1-Prepare Mousse: melt the dark chocolate chips and mix with warm water and cocoa powder, while separately melting the white chocolate chips with some heavy cream. Whip the remaining heavy cream with powdered sugar, vanilla, and salt to stiff peaks, then fold half into the dark chocolate mixture and the other half into the white chocolate mixture. Cover and chill for at least 2 hours to set.

2-Make Ganache: heating heavy cream until it bubbles, then pour it over dark chocolate chips and corn syrup if you’re using it. Let it sit for a bit, stir until smooth, add butter and salt, then cover and cool to room temperature.

3-Bake Cake: preheat your oven to 350°F (177°C) and mix the dry ingredients in a bowl. Cream the butter and gradually add egg whites, then mix in buttermilk, oil, vanilla, and espresso if desired, to form a smooth batter. Pour the batter into greased and lined pans and bake for 40-45 minutes for a 9×13 pan or 34-37 minutes for two 9-inch pans. After baking, cool for 10 minutes, loosen the edges, freeze for 30 minutes to firm, level the layers, and thaw for 20 minutes if needed. If you choose to use the optional simple syrup, dissolve sugar in boiling water and whisk in cocoa powder, then cool it before brushing on the cake layers to keep them moist.

4-Assemble: placing the first layer on a board with a bit of ganache, brush with simple syrup if desired, spread dark chocolate mousse, add white chocolate mousse, top with the second layer, brush again, and tidy the edges. Chill for 15 minutes, then cover with ganache and decorate as you like. Serve chilled or let it rest up to an hour at room temperature, and store it covered in the fridge for up to 3 days or freeze for a month.