Line two 9-inch cake pans with plastic wrap. Ensure the plastic is large enough to cover the base and sides.
In a double boiler over medium heat, melt the dark chocolate, butter, and Kahlua together. Alternatively, microwave in 30-second intervals. Mix in salt and vanilla and set aside to cool slightly.
In a stand mixer, whisk egg whites on medium speed until foamy. Gradually add sugar and whip until medium peaks form.
Bloom gelatin in water for at least 5 minutes.
Whip egg yolks until pale and fold into the chocolate mixture.
Heat 1/2 cup of heavy cream in the microwave until warm but not boiling. Add the gelatin-water mixture and mix. Fold into the chocolate mixture.
Fold whipped egg whites into the chocolate mixture.
Whip remaining 1 cup of heavy whipping cream until soft peaks form and fold into the chocolate mixture.
Divide mousse between the prepared cake pans and chill for at least 4 hours or overnight.
**Black Magic Cake:**
Preheat the oven to 350°F. Grease and flour three 9-inch round cake pans.
In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt.
Add vanilla, eggs, buttermilk, and oil. Mix until combined.
Stir in hot coffee until the batter is smooth and thin.
Pour the batter into the prepared pans and bake for 30-40 minutes or until a toothpick comes out clean.
Let the cakes cool completely.
**Ganache:**
In a bowl, combine chocolate, butter, vanilla, and salt.
In a small pan, heat cream and sugar until hot but not boiling. Pour over the chocolate mixture and let sit for 1 minute.
Stir until smooth. Chill in the fridge until slightly thickened but not hard.
**Assembly:**
Place a cake layer on a plate or cake stand. Top with a layer of mousse (use the plastic wrap to lift and flip the mousse onto the cake).
Repeat with another cake layer and mousse layer. Top with the final cake layer.
Gently carve a slight indent around the mousse layers with a spoon to keep the mousse inside the cake.
Ice the cake with chocolate ganache. Serve immediately or store in the fridge until ready to serve.