Preheat your oven to 350°F (175°C). Grease two 9-inch round cake pans with butter and line the bottoms with parchment paper.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, Dutch-processed cocoa powder, baking powder, and salt until well blended.

In a separate bowl, blend the butter, egg whites, buttermilk, vegetable oil, vanilla extract, and optional instant espresso until smooth and creamy.

Gradually add the wet ingredients to the dry mix, gently folding until just combined. Divide the batter between the prepared cake pans and smooth the tops. Bake for 30-35 minutes or until a toothpick comes out clean.

Allow the cake layers to cool in the pans for about 10 minutes, then transfer them to wire racks to cool completely.

For the white chocolate mousse, melt white chocolate chips with half of the warm water in a double boiler, then cool slightly. Whip the heavy cream with powdered sugar until soft peaks form and fold in the cooled chocolate.

For the dark chocolate mousse, melt dark chocolate chips with the remaining warm water, cool, and then fold into whipped cream with powdered sugar until combined. Chill for at least 2 hours.

To make the ganache, heat 1 cup of heavy cream in a saucepan until simmering. Pour over dark chocolate chips, let it sit before stirring until smooth. Mix in butter, light corn syrup, and pinch of salt.

Level the tops of the cooled cake layers. Spread a generous layer of white chocolate mousse on the first layer, add the second layer on top with dark chocolate mousse, and smooth it evenly.

Add the third layer on top and pour the ganache over the entire cake. Chill for at least 1 hour.

Slice using a hot knife for clean cuts and serve the cake to enjoy.