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 the flour, sugar, cocoa powder, baking powder, baking soda, and salt until well combined.

In a separate bowl, combine the 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. Don’t worry; this thin consistency creates the cake’s tender crumb.

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

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

For the chocolate mousses, heat 1 cup of heavy cream in a saucepan until it begins to simmer. Remove from heat immediately.

In a bowl, whisk the egg yolks with sugar and cornstarch until pale and thick. Slowly pour the hot cream into the yolk mixture, whisking constantly to prevent curdling.

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

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

To assemble, place one cake layer on your serving plate. Spread the dark chocolate mousse evenly over the top, then add the second cake layer.

Spread the white chocolate mousse over the top layer, creating smooth, even coverage. Refrigerate for at least 2 hours to set.

For the ganache, heat the heavy cream until it begins to simmer, then pour it over the milk chocolate. Let sit for 2 minutes, then stir until smooth and glossy.

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