1. Preheat your oven to 350°F. Grease and line three 9-inch round pans with parchment paper. In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt. In a separate bowl, beat the butter until smooth, then add the egg whites, buttermilk, oil, vanilla, and espresso (if using) and mix until well combined. Gently fold the wet ingredients into the dry ingredients just until they’re combined. Divide the batter evenly among the prepared pans and bake for 30-35 minutes, until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for 10 minutes, then invert them onto wire racks to cool completely.

2. In a double boiler or microwave, melt the white-chocolate chips with the warm water, stirring until smooth. Allow the mixture to cool to lukewarm. In a chilled bowl, whip the heavy cream with the powdered sugar until soft peaks form. Gently fold the melted white chocolate mixture into the whipped cream until well combined. Cover and chill the 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 heavy cream with the powdered sugar until soft peaks form. Fold the melted dark chocolate into the whipped cream until well combined. Cover and chill the mousse for at least 2 hours.

4. In a saucepan, heat the heavy cream until it just begins to simmer. Remove the pan from the heat and pour the hot cream over the dark-chocolate chips. Let the mixture sit for 2 minutes, then stir until the chocolate is completely melted and the ganache is smooth. Add the butter, corn syrup, and salt, and stir until well combined. If the consistency needs adjusting, you can add a small amount of warm cream to thin it out.

5. Level the three cake layers by trimming the tops to create flat, even surfaces. Place the first cake layer on a cake stand or serving plate. Spread the white-chocolate mousse evenly over the top. Add the second cake layer and spread the dark-chocolate mousse over it. Top with the third cake layer. Chill the assembled cake for about 15 minutes to firm up the mousse layers.

6. Once the cake is chilled, pour the chocolate ganache over the center, allowing it to drip down the sides. Use a spatula or bench scraper to smooth the ganache and create a clean, polished look.