Bumpy Cake

A soft chocolate sheet cake topped with fluffy vanilla buttercream ridges and glossy hot fudge frosting. This nostalgic Detroit-style Bumpy Cake is rich, tender, and easier to make than it looks.

Course Dessert

Cuisine American

Keyword chocolate cake recipes

Prep Time 25

Cook Time 35

Cooling Time 1 20

Total Time 2 20

Servings - 24 servings

Calories 546kcal

Author Kathleen

Ingredients

For the Cake:

1/2 cup 2oz, 60g unsweetened cocoa powder
1 1/2 teaspoons baking soda
1/2 teaspoon table salt
1/2 cup 4fl.oz, 125ml brewed coffee hot
1/2 cup 4fl.oz, 125ml vegetable or canola oil
1 cup 8fl.oz, 250ml buttermilk
2 teaspoons vanilla extract
2 eggs room temperature
1 3/4 cups 12.3oz, 350g granulated sugar
2 cups 10.6oz, 300g all purpose flour

For the Buttercream:

6 tablespoons unsalted butter softened
1/8 teaspoon table salt
2 cups 9.2oz, 260g powdered sugar
1 teaspoon vanilla extract
1-2 tablespoons buttermilk

For the Hot Fudge Frosting:

1/2 cup 4fl.oz, 125ml buttermilk
2 tablespoons corn syrup
1/2 cup 2oz, 60g unsweetened cocoa powder
1/2 cup 4oz, 115g unsalted butter
1/4 teaspoon table salt
4 cups 18.3oz, 520g powdered sugar
1 teaspoon vanilla extract

Instructions

Prep the pan. Preheat your oven to 350°F (177ºC). Spray a 9x13-inch cake pan with nonstick spray and set aside.
Bloom the cocoa. In the bowl of a stand mixer, whisk together the cocoa powder, baking soda, and salt. Pour in the hot coffee and mix on low speed for about 15–20 seconds until smooth.
Add the wet ingredients. Add the oil and buttermilk and mix until combined. Add the vanilla and eggs, then beat on medium speed until smooth and well blended, about 1–2 minutes.
Finish the batter. Add the sugar and flour and mix on medium speed just until smooth and no flour streaks remain. The batter will be thinner than typical cake batter — that’s completely normal.
Bake the cake. Pour the batter into the prepared pan and bake for 30–35 minutes, or until the center springs back lightly when touched and loses its shiny appearance. A toothpick inserted into the center should come out with a few moist crumbs.
Cool and chill. Let the cake cool in the pan until comfortable to handle, about 20–30 minutes. Transfer the pan to the freezer and chill for 20 minutes.
Make the buttercream. While the cake chills, beat the butter on high speed until smooth and creamy. Add the salt, powdered sugar, vanilla, and 1 tablespoon buttermilk. Start mixing on low speed, then gradually increase to medium-high and beat until thick and fluffy. Add the second tablespoon of buttermilk if needed to smooth the frosting.
Pipe the bumps. Transfer the buttercream to a piping bag fitted with a large round tip. Pipe thick horizontal ridges across the short side of the cake, leaving about 1/2 to 1 inch between each row.
Freeze the ridges. Return the cake to the freezer for about 40 minutes, or until the buttercream ridges feel very firm.
Make the fudge frosting. In a medium saucepan, combine the buttermilk, corn syrup, cocoa powder, butter, and salt. Cook over medium-low heat, stirring constantly, until the butter melts and the mixture just begins to bubble.
Finish the frosting. Remove the pan from the heat and whisk in the powdered sugar and vanilla until smooth and glossy. Let the frosting cool slightly while the cake finishes chilling.
Frost the cake. Remove the cake from the freezer. Slowly spoon the warm fudge frosting over the buttercream ridges and cake, covering the bumps completely. Gently tilt the pan to help the frosting spread evenly instead of using a spatula.
Chill and serve. Refrigerate the cake until the frosting is set. For the best texture, let slices sit at room temperature for about 15–20 minutes before serving.

Notes

Use room-temperature eggs: They blend more smoothly into the batter and help create a softer, more even cake texture.
Don’t skip the freezer time: The buttercream bumps need to be very cold before the warm fudge frosting goes on top.
Let the fudge frosting cool slightly: If it’s too hot, it can melt or flatten the buttercream ridges.
Avoid overbaking: This cake should be soft, tender, and springy — not dry.
Pipe the buttercream generously: Taller ridges create the classic bakery-style Bumpy Cake look.

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