Preheat your oven to 350°F (175°C). Grease and line three 9-inch round cake pans with parchment paper. In a large mixing bowl, combine dry ingredients: flour, sugar, cocoa powder, baking powder, and salt.

In another bowl, whisk wet ingredients: melted butter, buttermilk, oil, egg whites, and vanilla extract. Mix both mixtures until just combined. Divide the batter among the prepared pans and bake for 30 to 35 minutes, until a toothpick comes out clean.

Allow the cakes to cool in the pans for 10 minutes before transferring to wire racks.

For the white chocolate mousse, melt white chocolate chips with a tablespoon of warm water, set aside to cool. In a chilled bowl, whip heavy cream and powdered sugar until soft peaks form. Fold in the cooled chocolate mixture and refrigerate for at least 2 hours.

For the dark chocolate mousse, melt dark chocolate chips in the same manner. Allow to cool slightly and whip heavy cream with powdered sugar until soft peaks form. Fold in the melted dark chocolate and refrigerate for at least 2 hours.

Heat heavy cream in a saucepan until simmering. Pour cream over dark chocolate chips, let sit for 2 minutes. Stir until smooth, then mix in room temperature butter and corn syrup. Let cool for 15 to 20 minutes.

Level the cooled cake layers with a serrated knife. Place a layer on a cake plate, spread white chocolate mousse, add a second layer with dark chocolate mousse, and finish with the final cake layer. Refrigerate for 30 minutes.

Pour cooled ganache over the cake, allowing it to cascade down the sides. Decorate as desired.