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 these wet ingredients until the espresso dissolves completely.

Gradually add the wet ingredients to the dry ingredients, mixing until just combined. The batter will look thick at this point.

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 them out onto wire racks to cool completely.

Heat 1 cup of heavy cream in a saucepan until it just simmers. Remove from heat.

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.

Return the mixture to the saucepan and cook over medium heat, stirring until it thickens enough to coat the back of a spoon (about 2-3 minutes).

Divide this custard base between two bowls. Add 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. Fold half into each chocolate mixture to create two distinct mousses.

Place one cake layer on a serving plate. Spread the dark chocolate mousse over the top, then add the second layer.

Spread white chocolate mousse over the top layer, covering it smoothly.

Refrigerate for at least 2 hours to set.

Heat the heavy cream until it simmers, then pour over the milk chocolate.

Let sit for 2 minutes, then stir until smooth.

Pour the ganache over the chilled cake, allowing it to drip down the sides for a classic tuxedo effect.

Refrigerate for another 30 minutes before serving.