Preheat your oven to 350°F (175°C) and grease three 9-inch round cake pans. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, dark cocoa powder, baking powder, and salt until well combined.

In a separate bowl, mix the room temperature unsalted butter, egg whites, buttermilk, vegetable oil, vanilla extract, and optional espresso powder until smooth. Combine the wet and dry ingredients, mixing until just combined.

Divide the batter evenly among the pans and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to cool completely in the pans.

While the cake layers cool, prepare the white chocolate mousse by melting the white chocolate chips with a small amount of warm water in a heatproof bowl over simmering water, stirring until smooth. Allow the mixture to cool to room temperature.

In a separate chilled mixing bowl, whip the heavy cream with powdered sugar, vanilla extract, and a pinch of salt until soft peaks form. Gently fold the cooled white chocolate into the whipped cream until fully incorporated. Refrigerate the mousse for at least 2 hours to set.

Next, prepare the dark chocolate mousse by melting the dark chocolate chips with warm water in a heatproof bowl over simmering water, stirring until fully melted and smooth. Allow it to cool slightly.

In another chilled bowl, whip the heavy cream with powdered sugar and a pinch of salt until soft peaks form. Fold the cooled dark chocolate mixture into the whipped cream gently, ensuring not to deflate the mixture. Refrigerate for at least 2 hours to allow it to firm up.

For the ganache, heat the heavy cream in a small saucepan until just simmering. Pour it over the remaining dark chocolate chips in a medium bowl; let it sit undisturbed for a couple of minutes.

After resting, add a pinch of salt and the corn syrup, stirring until the mixture is completely smooth and glossy. Allow the ganache to cool slightly before using, as it will thicken as it cools.

Once the cake layers have cooled completely, check for evenness—level them if necessary. Place the first layer on a serving plate and spread a generous layer of white chocolate mousse on top.

Gently place the second layer on top and spread it with dark chocolate mousse. Finally, add the third cake layer and chill the assembled cake in the refrigerator for about 30 minutes.

Remove the chilled cake from the refrigerator and pour the ganache evenly over the top, allowing it to drip down the sides. Use an offset spatula to spread the ganache for even coverage.

If you have extra mousse, feel free to pipe some on top of the cake for decoration. Optionally, add chocolate shavings or mint leaves for a touch of elegance.

Chill the finished Chocolate Tuxedo Cake for at least 2 hours before serving.