Preheat your oven to 350°F (175°C) and prepare two 9-inch round cake pans lined with parchment paper.

In a large mixing bowl, whisk together all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt.

Mix together whole milk, eggs, sour cream, vegetable oil, espresso powder, and vanilla extract until smooth.

Gradually pour the wet ingredients into the dry ingredients, stirring gently. Add hot water and mix until smooth.

Divide the batter between prepared pans and bake for 30-35 minutes. Cool in pans for 10 minutes before transferring to wire racks.

Heat heavy cream until simmering. Whisk egg yolks with sugar and cornstarch until light, then combine with hot cream to temper.

Cook the tempered mixture over low heat, stirring until it thickens. Divide this between two bowls, mixing dark chocolate into one and white chocolate into the other.

Whip the remaining heavy cream to soft peaks, then fold half into the dark chocolate mix and half into the white chocolate mix.

Place one cake layer on your serving platter and spread dark chocolate mousse over it. Top with the second cake layer and spread with white chocolate mousse. Refrigerate for at least 2 hours.

For ganache, heat cream until simmering and pour over milk chocolate. Stir until smooth and let cool slightly before using.

Pour ganache over the top of the assembled cake and refrigerate for 30 minutes to set.

Slice the cake and serve with fresh berries or a dusting of powdered sugar.