Preheat your oven to 350°F (180°C) and grease two 8-inch cake pans.

In a large mixing bowl, sift together the dry ingredients: flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk to fully combine.

(2 cups all-purpose flour, 2 cups granulated sugar, ½ cup cocoa powder, 1 ½ teaspoons baking powder, 1 ½ teaspoons baking soda, ½ teaspoon salt)

In a separate bowl, whisk together the wet ingredients—milk, eggs, sour cream, vegetable oil, and vanilla extract—until smooth and uniform.

(½ cup whole milk, 2 large eggs, ½ cup sour cream, ½ cup vegetable oil, 1 teaspoon vanilla extract)

Add the wet mixture to the dry ingredients and whisk until no lumps remain.

In a small bowl or cup, mix the hot water with the espresso powder until fully dissolved, then stir this into the batter until smooth.

(1 teaspoon espresso powder, 1 cup hot water)

Divide the batter evenly between the two prepared pans and bake for about 35 minutes, or until a toothpick inserted in the center comes out clean.

Allow the cakes to cool completely in the pans before removing. Once cooled, level the tops using a serrated knife or cake leveler. If desired, slice the cake layers horizontally to make thinner layers.

Place a cake ring on a cake board or serving plate and set one of the cake layers inside the ring. Set aside while you prepare the mousse layers.

Finely chop the dark and white chocolate and place them in two separate heatproof bowls.

(3.5 oz dark chocolate, 4.5 oz white chocolate)

In another bowl, whisk the egg yolks with sugar and cornstarch until pale and slightly thickened.

(4 large egg yolks, 2 tablespoons granulated sugar, 2 tablespoons cornstarch)

Divide the heavy cream into two equal portions (1 cup / 240ml each). Set one portion aside for whipping later.

(2 cups heavy cream total, 1 teaspoon vanilla)

Pour the remaining cream into a saucepan with the vanilla and heat over medium-low until just steaming with tiny bubbles on the surface—do not let it boil.

Slowly drizzle some of the hot cream into the egg yolk mixture, whisking constantly to temper the eggs. Gradually add all the hot cream, then return the mixture to the saucepan.

Cook over medium-low heat, stirring constantly, until it starts to bubble. Let it boil gently for 1–2 minutes until thickened.

Remove from the heat and divide the custard evenly between the bowls of chopped chocolate. Stir each until the chocolate is fully melted and the mixtures are smooth.

In a separate bowl, whip the reserved cream to stiff peaks. Divide it in half and gently fold each half into the dark and white chocolate bases to create the mousse layers.

Pour the dark chocolate mousse over the cake layer inside the ring and smooth the top.

Carefully pour the white chocolate mousse over the dark layer, smoothing the surface. If the dark mousse is too liquid and the layers begin to blend, chill the cake for a few minutes before adding the white layer.

Refrigerate the cake for 30 minutes. Afterward, gently place the second cake layer on top and return the cake to the fridge for at least 4 hours to fully set.

After the cake has chilled for 4 hours, release it from the cake ring and return it to the fridge while preparing the ganache.

Finely chop the milk chocolate and place it in a bowl.

(5 oz milk chocolate)

In a small saucepan, heat the cream until it just begins to steam with small bubbles forming—don’t let it boil. Remove from heat and pour it over the chocolate.

(⅓ cup heavy cream)

Let the mixture sit for 1 minute, then stir until smooth, shiny, and completely melted. Allow the ganache to cool to room temperature. It should be thick enough to stay on top of the cake without dripping, but fluid enough to spread evenly.

Spread the ganache over the top of the cake using an offset spatula or knife, smoothing it out to the edges. Decorate with sprinkles, chocolate shavings, or any desired toppings.