Chocolate cake becomes something special when its smooth surface is transformed into a series of creamy ridges and covered with a shiny layer of fudge frosting. Detroit bumpy cake is known for exactly that combination.

The cake underneath is soft and deeply chocolatey, while the chilled buttercream creates the signature raised pattern beneath the final coating. It takes a little patience because several chilling stages are part of the process, but each step has a clear purpose.

Ingredients

250g plain flour

40g cornflour

1½ tsp bicarbonate of soda

100g dark cocoa powder

2 tbsp instant espresso powder

250ml boiling water

100g plant-based baking spread, softened

200g caster sugar

200g light brown soft sugar

2 large eggs, at room temperature

2 tsp vanilla extract

250ml buttermilk, at room temperature

55g unsalted butter, softened

55g plant-based baking spread, softened

225g icing sugar, sifted

1 tsp vanilla extract

5 tbsp double cream

400g granulated sugar

60g liquid glucose or corn syrup

50g cocoa powder

120g unsalted butter

1 tsp vanilla extract

Preparation and Cooking Time

Preparation takes about 1 hour, followed by around 30–35 minutes of baking. Allow at least 2 hours for cooling and chilling before serving. The recipe makes approximately 15 portions.

Instructions

1. Heat the oven to 180C, or 160C fan. Grease and line a 22 x 32cm baking tin.

2. Sift the flour, cornflour and bicarbonate of soda into a bowl. Mix the cocoa and espresso powder with the boiling water, then let it cool.

3. Beat the baking spread with both sugars until lighter. Add the eggs one at a time, followed by the vanilla.

4. Mix in the cooled chocolate mixture. Add the dry ingredients alternately with the buttermilk, mixing just until combined.

5. Spread the batter evenly in the tin and bake for 30–35 minutes, until a skewer comes out clean. Cool for 30 minutes, then freeze for 1 hour.

6. For the buttercream, beat the butter, baking spread and icing sugar until smooth. Add the vanilla and cream, then beat again.

7. Pipe several thick, evenly spaced buttercream lines across the chilled cake. Freeze for another 30 minutes.

8. For the fudge topping, heat the sugar, water, syrup, cocoa powder and a pinch of salt in a saucepan until the sugar dissolves. Add the butter and vanilla.

9. Bring the mixture to a boil without stirring and cook until it reaches about 112C. Let it cool slightly while keeping it pourable.

10. Pour the fudge topping over the chilled buttercream ridges. Chill briefly until set, then bring the cake to room temperature before slicing.

What Makes the Cake Different?

The defining feature is the contrast between the cake, buttercream and fudge topping. Instead of spreading buttercream across the entire surface, it is piped into individual ridges. The cold cake helps those ridges remain distinct while the warm frosting flows over them.

The final topping also needs careful attention. It should be thick enough to cover the buttercream but fluid enough to spread across the surface. Temperature plays an important role, which is why the cake is chilled before the frosting is added.

Helpful Baking Tips

Keep the eggs and buttermilk at room temperature when possible. This helps the batter combine more evenly. Avoid overmixing once the flour has been added, as excessive mixing can make the finished cake less tender. The chilling stages should not be rushed. A soft cake or warm buttercream can cause the raised lines to lose their shape. If the frosting begins setting before it has covered the cake, work efficiently while it is still glossy and pourable.

Detroit bumpy cake is a rewarding project for anyone who enjoys layered chocolate desserts. Its appeal comes from texture as much as flavour: a tender chocolate base, creamy buttercream ridges and a smooth fudge coating create several distinct layers in every slice. With careful chilling and attention to the frosting temperature, the unusual appearance becomes much easier to achieve at home.