01 -
Grease two 9-inch round cake pans and line the bottoms with parchment paper. Preheat oven to 350°F.
02 -
Whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl until fully combined.
03 -
In a separate bowl, whisk milk, eggs, sour cream, vegetable oil, espresso powder, and vanilla extract until espresso powder is dissolved.
04 -
Gradually add wet ingredients to dry ingredients, mixing gently until combined. Then slowly stir in hot water until batter is smooth and thin.
05 -
Divide batter evenly between prepared pans and bake for 30-35 minutes or until a toothpick comes out clean.
06 -
Allow cakes to cool in pans for 10 minutes before transferring to wire racks to cool completely, about 2 hours.
07 -
Heat 1 cup heavy cream until it just begins to simmer, then remove from heat.
08 -
In a bowl, whisk egg yolks, sugar, and cornstarch until pale and thick. Slowly whisk in hot cream to prevent curdling.
09 -
Return mixture to saucepan and cook over medium heat, stirring constantly until thick enough to coat back of a spoon (2-3 minutes).
10 -
Separate custard equally into two bowls. Stir dark chocolate into one and white chocolate into the other until smooth.
11 -
Whip remaining 1 cup heavy cream to soft peaks and gently fold half into each chocolate custard to create two mousses.
12 -
Place one cake layer on serving plate, spread dark chocolate mousse evenly over top, then place second cake layer.
13 -
Spread white chocolate mousse evenly over top layer and refrigerate for at least 2 hours to set.
14 -
Heat heavy cream until simmering and pour over milk chocolate. Let stand 2 minutes, then stir until smooth and glossy.
15 -
Pour ganache over chilled cake allowing drips down sides. Refrigerate an additional 30 minutes before serving.