Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans. In a large mixing bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt until well combined. In another bowl, mix sugar, milk, eggs, sour cream, oil, espresso powder, and vanilla extract. Combine wet and dry ingredients, then stir in hot water until smooth.

Divide the batter evenly between the two pans and bake for 30 to 35 minutes, or until a toothpick inserted comes out clean. Let the cakes cool for about 10 minutes, then turn them out onto a wire rack.

In a saucepan, heat 1 cup of heavy cream over medium heat until just simmering. Whisk together egg yolks and sugar until pale. Gradually stir hot cream into yolks, return to saucepan, cook until thick. Remove from heat and mix in chopped dark chocolate until melted. Let it cool slightly and fold in whipped cream until smooth.

Repeat the process for the white chocolate mousse by heating another cup of heavy cream until simmering. Whisk yolks and sugar, temper with hot cream, and cook until thick. Stir in finely chopped white chocolate until melted, then fold in whipped cream.

Place one cooled cake layer on a serving plate. Spread a layer of dark mousse over the top. Place the second cake layer on top of the mousse, pressing gently. Spread the white chocolate mousse evenly on top and refrigerate for at least 2 hours.

To create the ganache, heat 1 cup of heavy cream until simmering, pour over chopped milk chocolate, let sit for 5 minutes before stirring until smooth. Let it cool slightly to a pourable consistency.

Once the ganache is ready, pour it evenly over the chilled cake, allowing it to drip down the sides. Chill for an additional 30 minutes to set the ganache.