Line 2 8” cake pans with plastic wrap and set aside. Note the mousse is going to be divided into two and poured into the pans so make sure the plastic is large enough to cover the full base and up the edges.
Add the chopped dark chocolate, butter, and Kahlua into a bowl. In a double boiler over medium heat, melt together the mixture. (This can also be done in a microwave in 30-second intervals). Mix in the salt and vanilla and set aside to cool slightly.
In a stand mixer or with a hand mixer, add the egg whites and whisk together on medium speed until just foamy. After the egg whites are foamy, add in the sugar and whip until medium peaks form.
Add the gelatin to the water. Mix to combine and set aside to bloom for at least 5 minutes.
While the gelatin is blooming, whip the egg yolks in a bowl with a hand mixer or the base of a stand mixer until they are pale in color. Fold the whipped egg yolks into the chocolate mixture.
After the yolks are added, heat ½ cup of heavy cream in the microwave until warm but not boiling. Add the gelatin-water mixture to the warmed cream and mix to combine. Fold the mixture into the chocolate mixture.
After the gelatin is added, fold the whipped egg whites into the chocolate mixture
Lastly, whip the remaining 1 cup of heavy whipping cream until soft peaks form. Fold the whipped cream into the chocolate mixture until combined.
Pour the mousse in the two prepared plastic lined cake pans, pouring half in each. Spread into an even layer so the top is flat. Set for at least 4 hours in the fridge or overnight
When it has been at least 4 hours and it is time to bake the cake layers, remove from the cake pans (we like to lift it with the plastic wrap and place it plastic side down on a plate) and place the mousse layers back in the fridge until assembly time