Prepare the Cake Batter: Preheat your oven to 350°F (175°C). In a large mixing bowl, whisk together the dry ingredients: all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt until well combined. In another bowl, whisk together the wet ingredients: whole milk, eggs, sour cream, vegetable oil, espresso powder, and vanilla extract. Gradually mix the wet ingredients into the dry until just combined, then stir in hot water for a smooth, thin batter.

Bake the Cake Layers: Divide the cake batter evenly between two greased 9-inch round cake pans. Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Once baked, remove the cakes from the oven and let them cool in the pans for 10 minutes. Carefully transfer the cakes to wire racks to cool completely.

Prepare the Chocolate Mousses: For the mousses, start by heating heavy cream in a saucepan until it begins to simmer. In a separate bowl, whisk together egg yolks, granulated sugar, and cornstarch until smooth. Gradually add the hot cream to the yolk mixture, whisking constantly to avoid scrambling. Cook this mixture over low heat until it thickens and coats the back of a spoon, about 5 minutes. Divide the custard into two bowls, adding melted dark chocolate to one and white chocolate to the other.

Whip the Cream and Combine: In a separate bowl, whip the remaining heavy cream until soft peaks form. Fold half of the whipped cream gently into the dark chocolate mixture until incorporated, then repeat with the white chocolate mixture.

Assemble the Cake: Place one cooled cake layer on a serving plate. Spread the dark chocolate mousse evenly on top, ensuring it reaches the edges. Carefully place the second cake layer on top of the mousse. Spread the white chocolate mousse over the top layer and smooth it out, then refrigerate for at least 2 hours to set.

Create the Ganache Topping: While the cake chills, prepare the ganache. Heat additional heavy cream in a saucepan until it simmers, then pour it over the chopped milk chocolate in a bowl. Allow it to sit for 5 minutes, then whisk until smooth and glossy.

Finish the Cake and Serve: Once the cake has chilled and the mousse is set, carefully pour the ganache over the top, allowing it to drizzle down the sides. Refrigerate for another 30 minutes to ensure everything is firm. When ready to serve, slice with a warm knife for clean edges.