Go Back Email Link
–+ servings
Ella Cooks

Japanese Sweet Potato Chocolate Cake (Fudgy, Purple-Crumbed, and Surprisingly Easy)

Love it? Click to rate
Be the first to rate this recipe!
A tender, fudgy chocolate layer cake made with roasted Japanese sweet potato. The mashed potato keeps the crumb moist and gives it a faint purple tint that looks beautiful against the dark ganache. Easy enough for a weeknight baker, impressive enough for any occasion.
Print Recipe Pin Recipe
Prep Time 20 minutes
Cook Time 30 minutes
Cooling Time 45 minutes
Servings: 10
Course: Dessert
Calories: 485
Ingredients Equipment Method Nutrition Notes

Ingredients
  

Ingredients — Cake
  • 300 g about 2 medium Japanese sweet potatoes, roasted and mashed
  • 200 g 1 cup granulated sugar
  • 100 g ½ cup light brown sugar, packed
  • 180 ml ¾ cup neutral oil (sunflower or vegetable)
  • 3 large eggs room temperature
  • 1 tsp vanilla extract
  • 180 g 1½ cups all-purpose flour
  • 60 g ¾ cup unsweetened cocoa powder (Dutch process preferred)
  • 1½ tsp baking soda
  • 1 tsp baking powder
  • ½ tsp salt
  • 180 ml ¾ cup buttermilk, room temperature
  • 180 ml ¾ cup hot strong coffee (or hot water)
Ingredients — Dark Chocolate Ganache
  • 300 g 10.5 oz dark chocolate (70%), finely chopped
  • 240 ml 1 cup heavy cream
  • 1 tbsp unsalted butter
  • Pinch of flaky sea salt

Equipment

  • 2 x 8-inch (20cm) round cake pans
  • Parchment paper
  • Large mixing bowl
  • Whisk
  • Rubber spatula
  • Cooling rack

Method
 

Method
  1. Roast the sweet potato: Preheat oven to 400°F (200°C). Pierce each sweet potato several times with a fork. Place on a baking sheet and roast for 40-50 minutes until completely soft and slightly caramelized at the edges. Cool, scoop out the flesh and mash until smooth. You need 300g of mashed potato. This can be done the day before and stored in the fridge.
  2. Prep: Reduce oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment.
  3. Make the batter: In a large bowl, whisk together the mashed sweet potato, both sugars, oil, eggs, and vanilla until smooth and combined. It will look a little curdled at this point - that's fine.
  4. Add dry ingredients: Sift in the flour, cocoa, baking soda, baking powder, and salt. Whisk until just combined - don't overmix.
  5. Add wet ingredients: Add the buttermilk and hot coffee. Whisk gently until a smooth, fairly liquid batter forms. It will be thinner than a standard cake batter.
  6. Bake: Divide batter evenly between the two prepared pans. Bake for 28-32 minutes until a toothpick comes out with just a few moist crumbs. Do not overbake - the sweet potato keeps the cake moist but it will dry out if left too long.
  7. Cool: Leave in pans for 10 minutes, then turn out onto a cooling rack. Cool completely before ganaching - at least 45 minutes. Rushing this will melt the ganache.
  8. Make the ganache: Place chopped chocolate and butter in a heatproof bowl. Heat cream in a small saucepan until just simmering (don't boil). Pour over the chocolate, leave for 2 minutes, then stir from the center outward until completely smooth. Add a pinch of flaky salt. Leave to cool at room temperature for 20-30 minutes until it thickens to a pourable but not runny consistency.
  9. Assemble: Place the first layer on your serving plate. Pour roughly a third of the ganache over the top and spread to the edges, letting it drip slightly. Place the second layer on top. Pour the remaining ganache over, spreading it across the top and down the sides as much as you like. Leave to set for at least 20 minutes before slicing.

Nutrition

Calories: 485kcalCarbohydrates: 58gProtein: 7gFat: 26gSaturated Fat: 10gFiber: 4gSugar: 38g

Notes

  • Can't find Japanese sweet potatoes? Look for them in Asian grocery stores or larger supermarkets - they have dark purple-red skin. Regular sweet potatoes work but the flavor is milder and the crumb slightly wetter.
  • Make ahead: Baked layers can be wrapped tightly and stored at room temperature for up to 2 days before assembling. Ganache can be made ahead and gently rewarmed.
  • Storage: Store assembled cake at room temperature under a cake dome for up to 3 days. Refrigerating will firm up the ganache - bring to room temperature before serving.
  • The coffee: You can't taste it in the finished cake, but it deepens the chocolate flavor significantly. Hot water works if you'd rather skip it.

Tried this recipe?

Let us know how it was!
QR Code linking back to recipe