Preheat your oven to 350°F (175°C) and grease three 9-inch round cake pans with unsalted butter, lining bottoms with parchment paper.

In a large mixing bowl, whisk together all-purpose flour, granulated sugar, dark cocoa powder, baking powder, and salt.

In a separate bowl, mix together unsalted butter, egg whites, buttermilk, vegetable oil, vanilla extract, and optional instant espresso powder until smooth.

Gradually pour the wet mixture into the dry ingredients, stirring gently until just combined.

Divide the batter among prepared pans and bake for 30-35 minutes, checking with a toothpick.

Cool cakes in pans for 10 minutes before transferring to wire racks.

Melt white chocolate chips with warm water, then cool slightly. Whip cold heavy cream with powdered sugar and salt, fold in white chocolate.

Melt dark chocolate chips with warm water, then cool. Whip heavy cream with powdered sugar and salt, fold in dark chocolate.

Heat heavy cream in a saucepan until simmering. Pour over dark chocolate chips and let sit, then stir in butter and corn syrup.

Level the cooled cake layers if needed. Layer cake with white chocolate mousse, then dark chocolate mousse, finishing with the last layer.

Chill the assembled cake in the refrigerator for about 30 minutes.

Pour ganache over the top of the chilled cake, letting it cascade down the sides. Optionally decorate with chocolate shavings or berries.

Refrigerate for at least 2 hours before serving.