Preheat oven to 350°F (175°C) and prepare two 9-inch cake pans by greasing and lining.

In a large bowl, whisk together all dry ingredients: flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, mix wet ingredients: milk, eggs, sour cream, vegetable oil, espresso powder, and vanilla until smooth.

Gradually combine wet ingredients with dry, then stir in hot water until batter is smooth. Divide into prepared pans.

Bake for 30-35 minutes until toothpick comes out clean. Cool in pans for 10 minutes before transferring to wire racks.

Heat heavy cream in a saucepan until simmering. Whisk yolks, sugar, and cornstarch until thick.

Gradually add hot cream to yolk mixture, whisking continuously, then return to saucepan and cook until thickened.

Divide custard into three parts; melt each chocolate and mix into corresponding custard to form mousses.

Whip remaining cream to soft peaks and fold into each mousse mixture gently.

Place first cake layer on a plate, spread dark chocolate mousse, top with second layer, and add white chocolate mousse.

Refrigerate cake for at least 2 hours to set mousse.

For ganache, heat cream until simmering, pour over milk chocolate and stir until smooth.

Pour ganache over the chilled cake, refrigerate again for 30 minutes before serving.