1. Preheat your oven to 350°F. Grease and line three 9-inch round pans with parchment paper. In a mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt. In a separate bowl, beat the butter, then add the egg whites, buttermilk, oil, vanilla, and espresso until smooth. Fold the wet ingredients into the dry just until combined. Divide the batter evenly among the prepared pans and bake for 30–35 minutes, or until a tester inserted comes out clean. Allow the cakes to cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.

2. In a double boiler or microwave-safe bowl, melt the white-chocolate chips with the 1/3 cup of warm water, stirring until smooth. Let the mixture cool to lukewarm. In a chilled bowl, whip the 1 1/3 cup of heavy cream with the powdered sugar until soft peaks form. Gently fold the melted white chocolate into the whipped cream. Chill the white-chocolate mousse for at least 2 hours.

3. Melt the dark-chocolate chips with the remaining warm water, stirring until smooth. Allow the mixture to cool slightly. In a chilled bowl, whip the 1 cup of heavy cream with the powdered sugar until soft peaks form. Fold the melted dark chocolate into the whipped cream. Chill the dark-chocolate mousse for at least 2 hours.

4. In a small saucepan, heat the 3/4 cup of heavy cream just until it starts to simmer. Remove from heat and pour the cream over the dark-chocolate chips. Let the mixture rest for 2 minutes, then stir until smooth. Add the butter, corn syrup, and salt, and stir until well combined. Adjust the consistency of the ganache by adding a little warm cream if needed.

5. Begin by leveling the three cake layers. Place the first layer on a cake stand or serving platter and spread the white-chocolate mousse evenly over the top. Add the second cake layer and spread the dark-chocolate mousse on top. Top with the third cake layer. Chill the assembled cake for about 15 minutes, then pour the chocolate ganache over the center, allowing it to drip down the sides.

6. Refrigerate the assembled Perfect Chocolate Tuxedo Cake for about 1 hour to firm up the layers. When ready to serve, use a hot, wiped knife to cut clean slices.