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.