Preheat oven to 350°F (175°C) and grease three 9-inch round cake pans with parchment paper.

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

In a separate bowl, blend unsalted butter, egg whites, buttermilk, vegetable oil, and vanilla extract until smooth.

Gradually combine wet and dry ingredients without overmixing.

Divide the batter among prepared pans and bake for 30-35 minutes.

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

Melt white chocolate chips in a heatproof bowl over simmering water or in microwave.

Allow melted chocolate to cool slightly.

Whip cold heavy cream with powdered sugar and vanilla extract until soft peaks form.

Fold cooled chocolate mixture into whipped cream and refrigerate for 2 hours.

Melt dark chocolate chips in a similar method as white chocolate.

Whip cold heavy cream with powdered sugar until soft peaks form.

Fold melted dark chocolate into whipped cream and refrigerate for 2 hours.

Heat heavy cream in a saucepan until simmering.

Pour hot cream over dark chocolate chips and let sit for a few minutes to melt.

Stir until smooth and glossy, then mix in softened butter and corn syrup if using.

Let ganache cool until slightly thickened.

Level the tops of the cake layers with a serrated knife before assembly.

On a serving plate, place the first cake layer and spread a generous layer of white chocolate mousse over the top.

Position the second layer on top and follow with dark chocolate mousse.

Add the third layer on top and chill the assembled cake for 30 minutes.

Pour the ganache evenly over the top and let it cascade down the sides of the cake.

Decorate with leftover mousse or chocolate shavings if desired.

Refrigerate the completed cake for a minimum of 2 hours before serving.