Preheat oven to 350°F (175°C) and prepare two 9-inch round cake pans by greasing them and dusting with flour.

In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, whisk the milk, eggs, sour cream, vegetable oil, espresso powder, and vanilla until creamy, then combine with dry ingredients.

Slowly add hot water to the batter until fully incorporated.

Divide the batter between the prepared pans and bake for 30-35 minutes until a toothpick comes out clean.

Let the cakes cool in the pans for 10 minutes, then transfer to wire racks to cool completely.

Heat 1 cup of heavy cream in a saucepan until it simmers. Whisk egg yolks, sugar, and cornstarch in a separate bowl.

Gradually pour hot cream into the egg yolk mixture, then return to the saucepan and cook until thickened.

Divide the mixture into three bowls. Stir in dark, white, and milk chocolate into each bowl until melted.

Whip the remaining heavy cream until soft peaks form, then fold into each chocolate mixture.

Assemble the cake by layering one cake with dark chocolate mousse, adding the second layer on top, then topping with white chocolate mousse.

For ganache, heat heavy cream, pour it over milk chocolate, and stir until smooth. Pour ganache over the assembled cake.