Begin by melting the dark chocolate, butter, and Kahlua or brewed coffee together in a double boiler over low heat. Stir gently until fully melted and combined, then allow the mixture to cool slightly while you prep your other ingredients.

In a clean mixing bowl, whip the egg whites on medium speed until they become foamy. Gradually add granulated sugar and continue whisking until medium peaks form, resembling soft clouds. This aeration will help create a light texture in the mousse, so set it aside carefully.

In a separate bowl, whip the egg yolks until they’re light and pale. Gently fold the cooled chocolate mixture into the yolks, ensuring to incorporate well without deflating the airiness. This forms the base of your mousse and enhances the rich chocolate flavor.

In a small dish, bloom the unflavored gelatin by sprinkling it over a few tablespoons of cold water and let it sit for about 5 minutes. After blooming, warm it gently to dissolve before adding it to the heavy whipping cream, stirring well before incorporating it into the chocolate mixture.

Next, gently fold in the whipped egg whites followed by the whipped cream into the chocolate mixture. Use a spatula to ensure you maintain the airy texture. Once combined, pour the mousse into lined cake pans and place them in the fridge to chill for at least 4 hours or overnight if possible.

Preheat your oven to 350°F (175°C). In a large bowl, mix all the dry ingredients: flour, sugar, cocoa powder, baking soda, baking powder, and salt. Add the eggs, buttermilk, oil, and hot coffee, mixing until smooth. Pour the batter into prepared pans and bake for 30-40 minutes until a toothpick inserted comes out clean. Allow the cakes to cool completely.

In a saucepan, heat the heavy cream and sugar over medium heat until hot, but not boiling. Pour this mixture over chopped dark chocolate and let it sit for a minute. Stir until smooth and glossy. Allow the ganache to cool slightly before using it to top the assembled cake.

Once everything is cooled, place one chocolate cake layer on a serving plate. Spoon a generous layer of mousse on top, followed by a second cake layer. Finally, pour the ganache evenly over the top and let it drizzle down the sides. Chill the assembled cake before slicing and serving.