I bought a bag of Okinawan purple sweet potatoes because I like eating them plain, just steamed. That habit is where this purple sweet potato casserole started.
Steamed purple sweet potato with a little sweetened condensed milk already feels like a treat. Bake it under a buttery pecan streusel and it’s a triple treat, the kind of side that belongs on a fall or holiday table.
It also looks like ube. That deep purple makes the whole dish feel a lot more special than the usual orange sweet potato casserole.
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Jump to:
- Why You’ll Love This Purple Sweet Potato Casserole
- What Is Purple Sweet Potato Casserole
- Ingredients You’ll Need
- Which Purple Sweet Potato to Buy
- Purple Sweet Potato vs Ube
- How to Make Purple Sweet Potato Casserole
- Rose’s Tips
- Storage and Reheating
- Recipe FAQs
- More Holiday Sides
- Did you try this recipe?
- Purple Sweet Potato Casserole
Why You’ll Love This Purple Sweet Potato Casserole
- It stands out on the table. The deep purple filling looks nothing like the orange casseroles everyone else brings, and it gets noticed before anyone takes a bite.
- Condensed milk does two jobs. It sweetens the filling and adds moisture to starchy Okinawan sweet potatoes, so there’s no extra sugar to measure in the base.
- Pecan streusel instead of marshmallows. A buttery, crunchy topping against the soft filling, and it bakes up golden in the same 30 minutes.
- Steamed, not boiled. Steaming keeps the potatoes from soaking up water, so the mash doesn’t turn loose or watery.
- A short ingredient list. Butter, an egg, cinnamon, and a can of condensed milk. Most of it is probably already in your pantry.
- Feeds a crowd from a small dish. One 10x7 baking dish serves 8.
What Is Purple Sweet Potato Casserole
Purple sweet potato casserole is a baked side dish made from mashed purple sweet potatoes, sweetened and enriched with butter and egg, then finished with a topping. It’s the same idea as a classic Thanksgiving sweet potato casserole, made with purple sweet potatoes instead of orange ones.
This version uses Okinawan sweet potatoes mashed with sweetened condensed milk, butter, egg, and cinnamon, baked under a brown sugar pecan streusel. If you’d rather go traditional, my old-fashioned sweet potato casserole is the orange version.
Ingredients You’ll Need
For the filling
- Okinawan purple sweet potatoes: They give the casserole its deep purple color and a mild, earthy sweetness. Steam them whole and peel after cooking, when the skin slides off easily. They’re starchier and drier than orange sweet potatoes, which is why the rest of the filling leans on moist, rich ingredients.
- Sweetened condensed milk: The only sweetener in the filling, and the main source of moisture. It’s thick enough to loosen the mash without making it runny. Don’t grab evaporated milk by mistake. It has no sugar, so the filling would taste flat.
- Melted butter: Adds richness and helps the mash whip up smooth. Melted mixes in faster than softened, especially into warm potatoes.
- Egg: Binds the filling so it bakes into a soft, scoopable casserole instead of a loose mash.
- Ground cinnamon: The warm holiday spice that makes this taste like a fall side dish. It also rounds out the condensed milk so the filling isn’t just sweet.
- Salt: A small amount keeps the condensed milk from tasting one-note.
For the pecan streusel
- Pecans, roughly chopped: The crunch on top, and the contrast that keeps a soft, sweet filling from feeling heavy. Rough pieces give you crunchy bites throughout instead of a fine, sandy topping.
- Brown sugar, packed: Its molasses gives the streusel a caramel flavor and helps it clump into crumbs.
- All-purpose flour: Gives the streusel body so it bakes into crisp clusters instead of melting into the filling. It’s the same idea as the crumb topping on my banana bread with streusel.
- Melted butter: Binds the flour, sugar, and pecans into crumbs and browns as it bakes.
- Salt: A pinch balances the brown sugar.
See the recipe card for quantities.
Which Purple Sweet Potato to Buy
Not every “purple” sweet potato is purple inside, so check before you buy.
- Okinawan sweet potatoes have beige or cream-colored skin and purple flesh. This is what I used.
- Stokes purple sweet potatoes have purple skin and purple flesh. They should work too, though I haven’t tested this recipe with them.
- Japanese sweet potatoes (also called murasaki) have purple skin but pale yellow or white flesh. They won’t give you a purple casserole.
Look for them at Asian grocery stores. Pick firm potatoes with no soft spots, since they’ll steam more evenly.
Purple Sweet Potato vs Ube
They look alike once they’re cooked, but they’re two different plants.
Ube is a purple yam. Its flesh is moister, and its flavor is often described as mellow and vanilla-like, which is why it shows up in so many Filipino desserts. In the US, fresh ube is hard to find. Most of what you’ll see is frozen grated ube, ube extract, or ube halaya in a jar.
Okinawan sweet potatoes are sweet potatoes. They have beige skin and purple flesh, and they cook up starchier and drier than ube or orange sweet potatoes, with a mild sweetness. They’re much easier to buy fresh, which is a big reason I reach for them.
So this casserole isn’t an ube casserole. It just shares the color. If ube is what you’re after, start with my ube halaya. For a holiday dish made with real ube, try my ube cassava cake.
How to Make Purple Sweet Potato Casserole
- Steam the sweet potatoes. Steam the potatoes whole, skin on, for 20 to 30 minutes, until a fork slides in easily. Larger potatoes take the longer end of that range.
- Peel. Let them cool just enough to handle, then peel. The skin slips right off after steaming. Preheat the oven to 350°F.
- Make the filling. Blend the warm potatoes in a food processor until smooth. Add the melted butter, sweetened condensed milk, cinnamon, and salt, and pulse until combined. Taste, then pulse in the egg.
- Fill the dish. Spread the filling evenly in an ungreased 10x7 baking dish. No need to grease it.
- Make the streusel. Stir together the chopped pecans, brown sugar, flour, and a pinch of salt. Pour in the melted butter and mix until it forms crumbs.
- Top and bake. Scatter the streusel over the filling and bake for 30 minutes, until the streusel is golden.
- Serve. Serve warm, straight from the oven.
Rose’s Tips
- Steam, don’t boil. Boiled sweet potatoes soak up water, and that water ends up in the filling. Steaming cooks them through while keeping the mash thick enough to hold its shape.
- Peel after steaming. The skin slides right off once the potatoes are cooked, which is faster and wastes less than peeling raw.
- Blend while the potatoes are warm. Warm potatoes blend up smooth. Once they cool, the starch firms up and the filling gets lumpy.
- Mix while the potatoes are warm. Warm potatoes whip up smooth. Once they cool, the starch firms up and the mash gets lumpy and harder to beat.
- Taste before the egg goes in. Purple sweet potatoes vary in sweetness. Tasting first lets you adjust the cinnamon or salt without eating raw egg.
- Chop the pecans roughly. Bigger pieces stay crunchy on top. Finely chopped pecans turn the streusel sandy.
Storage and Reheating
- Fridge: Cover leftovers and refrigerate for up to 3 to 4 days.
- Reheating: Warm individual portions in the microwave, or reheat the dish in a 350°F oven, loosely covered with foil so the streusel doesn’t over-brown. The topping softens in the fridge and won’t be as crunchy the second day.
Recipe FAQs
Okinawan sweet potatoes are starchier and drier than orange ones, so they need moisture as much as sweetness. Sweetened condensed milk handles both in one ingredient. It sweetens the filling and loosens the mash without adding so much liquid that it turns runny. Don’t swap in evaporated milk, which has no sugar.
I haven’t tested this recipe with them. Orange sweet potatoes are moister than Okinawan ones, so the filling would likely come out looser. If you want the orange version, my old-fashioned sweet potato casserole is written for them.
You can, but drain them very well and let them steam dry for a few minutes before mixing. Boiled potatoes hold more water, which can make the filling softer. I steam mine.
I used one, and it blends the filling completely smooth. A hand mixer or potato masher should also work, though I haven’t tested either, and a masher will leave a more rustic texture.
More Holiday Sides
- Old-Fashioned Sweet Potato Casserole
- Sausage Green Bean Potato Casserole
- Crack Green Beans
- Roasted Brussels Sprouts
- Sugar-Free Cranberry Sauce
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Purple Sweet Potato Casserole
Equipment
- Steamer
- Food processor
- MIxing Bowl
- 10x7 baking dish
Ingredients
Filling
- 623 grams Okinawan purple sweet potatoes about 4, or 1⅓ pounds
- 4 tablespoons salted butter melted
- ⅔ cup sweetened condensed milk
- 1 large egg
- 1 teaspoon ground cinnamon
- ⅛ teaspoon salt
Pecan streusel
- ½ cup pecans roughly chopped
- ¼ cup packed brown sugar
- ¼ cup all-purpose flour
- 3 tablespoons salted butter melted
- Pinch of salt
Instructions
- Steam the sweet potatoes whole, skin on, for 20 to 30 minutes, until a fork slides in easily.623 grams Okinawan purple sweet potatoes
- Let them cool just enough to handle, then peel. Preheat the oven to 350°F.
- Blend the warm potatoes in a food processor until smooth. Add the melted butter, condensed milk, cinnamon, and salt, and pulse until combined. Taste, then pulse in the egg.4 tablespoons salted butter, ⅔ cup sweetened condensed milk, 1 teaspoon ground cinnamon, ⅛ teaspoon salt, 1 large egg
- Spread the filling evenly in an ungreased 10x7 baking dish.
- Stir together the pecans, brown sugar, flour, and salt. Pour in the melted butter and mix until it forms crumbs.½ cup pecans, ¼ cup packed brown sugar, ¼ cup all-purpose flour, 3 tablespoons salted butter, Pinch of salt
- Scatter the streusel over the filling and bake for 30 minutes, until the streusel is golden.
- Serve warm, straight from the oven.
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Notes
- Sweet potatoes: Use Okinawan sweet potatoes, with beige skin and purple flesh. Purple-skinned Japanese sweet potatoes are pale inside and won’t give you the color.
- Condensed milk: Use sweetened condensed milk, not evaporated milk.
- Butter: I use salted butter. If you only have unsalted, add a small pinch more salt to the filling.
- Leftovers: Refrigerate covered for up to 3 to 4 days. Reheat loosely covered with foil so the streusel doesn’t over-brown.
