. Prepare the Chocolate Mousses:

Dark Chocolate Mousse:

1. Melt dark chocolate chips in the microwave in 30-second intervals until smooth.
2. Mix warm water with cocoa powder and combine with melted chocolate.
3. Whip 1 cup of heavy cream with powdered sugar and vanilla until stiff peaks form.
4. Fold whipped cream into the chocolate mixture. Chill for 2 hours.

White Chocolate Mousse:

1. Melt white chocolate chips with 3/4 cup heavy cream over low heat until smooth. Cool slightly.
2. Whip remaining heavy cream with powdered sugar, vanilla, and salt until stiff peaks form.
3. Fold whipped cream into the white chocolate mixture. Chill for 2 hours.

2. Make the Ganache:

1. Heat heavy cream until it starts to bubble but does not boil.
2. Pour over dark chocolate chips and let sit for 1 minute. Stir until smooth.
3. Mix in butter, corn syrup, and salt. Let cool to room temperature before using.

3. Bake the Cake Layers:

1. Preheat oven to 350°F (175°C). Grease and line three 8-inch round pans.
2. In a large bowl, combine sugar, flour, cocoa powder, baking powder, and salt.
3. Beat in butter until the mixture resembles wet sand.
4. Add egg whites and buttermilk, mixing until smooth. Stir in vegetable oil, vanilla extract, and instant espresso (if using).
5. Divide batter evenly among prepared pans.
6. Bake for 34-37 minutes or until a toothpick inserted in the center comes out clean.
7. Cool cakes completely on a wire rack.

4. Assemble the Cake:

1. Place one cake layer on a serving plate. Spread a layer of dark chocolate mousse on top.
2. Add the second cake layer and spread with white chocolate mousse.
3. Top with the final cake layer. Refrigerate for 15 minutes to set the mousses.
4. Pour ganache over the top, letting it drip down the sides for a polished look.
5. Decorate with additional ganache, mousse, or chocolate shavings.