Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans, lining the bottoms with parchment paper. In a large mixing bowl, whisk together the dry ingredients—flour, sugar, cocoa powder, baking powder, baking soda, and salt. In another bowl, combine the wet ingredients—milk, eggs, sour cream, and vegetable oil. Gradually stir the wet mixture into the dry ingredients, then mix in the hot water.

Divide the prepared batter evenly between the two pans and smooth the tops with a spatula. Place them in the oven and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for 10 minutes before transferring them to wire racks to cool completely for at least 2 hours.

Bring 1 cup of heavy cream to a simmer in a saucepan. In a separate bowl, whisk together the egg yolks, granulated sugar, and cornstarch until thick. Gradually add the hot cream to the egg mixture, whisking constantly. Pour it back into the saucepan and return to medium heat, stirring until thickened. Divide the mixture in half, stirring dark chocolate into one half and white chocolate into the other. Fold in whipped cream gently.

Place one layer of the cooled cake on a serving plate. Spread the dark chocolate mousse on top, then add the second layer and spread the white chocolate mousse over it. Cover the entire cake loosely with plastic wrap and chill it in the refrigerator for at least 2 hours.

Heat the remaining 1 cup of heavy cream until it simmers. Pour it over the chopped milk chocolate in a bowl and let it sit for a few minutes. Stir until smooth, then pour the ganache over the chilled cake, allowing it to drizzle down the sides.

Let the cake chill in the refrigerator for an additional 30 minutes to set the ganache before slicing. Use a warm knife to cut through the layers for a beautiful presentation.