Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans. Line the bottoms with parchment paper for easy removal.

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

In a separate bowl, combine milk, eggs, sour cream, vegetable oil, espresso powder, and vanilla extract. Whisk thoroughly to ensure the espresso dissolves.

Gradually add the wet ingredients to the dry ingredients, mixing until just combined. Slowly stir in the hot water until the batter is smooth and thin.

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

Cool the cakes in their pans for 10 minutes, then turn out onto wire racks to cool completely for at least 2 hours.

Heat 1 cup of heavy cream until it just begins to simmer. In a bowl, whisk egg yolks with sugar and cornstarch until pale and thick.

Slowly pour the hot cream into the yolk mixture, whisking constantly, then return it to the saucepan and cook until thickened (about 2-3 minutes).

Divide this custard base between two bowls. Add the dark chocolate to one bowl and white chocolate to the other, stirring until melted and smooth.

Beat the remaining 1 cup of heavy cream to soft peaks, then fold half into each chocolate mixture.

Place one cake layer on your serving plate. Spread the dark chocolate mousse evenly over the top and add the second cake layer.

Spread the white chocolate mousse over the top layer. Refrigerate for at least 2 hours to set.

Heat the heavy cream until it just begins to simmer, pour over the milk chocolate, let sit for 2 minutes, then stir until smooth.

Pour the ganache over the chilled cake, letting it drip down the sides for the classic tuxedo effect. Refrigerate for another 30 minutes before serving.