Preheat your oven to 350°F (175°C) and prepare three 9-inch round cake pans by greasing and flouring them.

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 unsalted butter, egg whites, buttermilk, vegetable oil, pure vanilla extract, and optional instant espresso powder until smooth.

Gradually fold the wet mixture into the dry ingredients, mixing on low until just combined.

Divide the batter evenly among the prepared cake pans and smooth the tops with a spatula.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Once baked, let the cakes cool in the pans for about 10 minutes before transferring them to wire racks to cool completely.

Melt the white chocolate chips with a tablespoon of warm water in a heatproof bowl over simmering water, stirring until smooth.

Remove from heat and let it cool slightly.

In another bowl, whip heavy cream until soft peaks form.

Gently fold the cooled white chocolate into the whipped cream until well combined.

Cover and refrigerate for at least 2 hours to allow the mousse to set.

Melt the dark chocolate chips with a tablespoon of warm water using the same method as before.

Once melted, remove from heat and let it cool.

In another bowl, whip more heavy cream until soft peaks form, then carefully fold in the cooled dark chocolate mixture.

Like the white mousse, refrigerate this mixture for at least 2 hours for the perfect texture.

Heat 1 cup of heavy cream in a saucepan until it just begins to simmer.

Pour the hot cream over 8 ounces of chopped dark chocolate in a mixing bowl.

Allow it to sit for 2 minutes, then stir in a tablespoon of unsalted butter and corn syrup until the mixture is smooth and glossy.

Let the ganache cool until it’s slightly thickened but still pourable, which will take around 15-20 minutes.

Once the cake layers are completely cooled, level the tops with a serrated knife if necessary.

Place the first layer on a serving plate and spread a generous layer of white chocolate mousse on top.

Add the second layer and spread dark chocolate mousse over that layer, then carefully place the final cake layer on top.

Chill the assembled cake in the refrigerator for 30 minutes to firm everything up.

After chilling, remove the cake from the refrigerator and pour the beautiful ganache over the top, allowing some to drip down the sides.

Optionally, create decorative flourishes with chocolate shavings or additional mousse.

Refrigerate the entire cake for at least 2 hours, or overnight, to let the flavors meld and the texture set before slicing.