In a heat-proof bowl set over a pan of simmering water, melt the two chocolates, coffee, coffee powder, Grand Marnier, Armagnac, and vanilla extract. Cool to room temperature. Beat in the softened butter.
Meanwhile, place the egg yolks and the 1/2 cup of sugar in the bowl of an electric mixer fitted with the paddle attachment. Beat on high speed for about 5 minutes, until pale yellow; when you lift the beater, the mixture will fall back on itself in a ribbon. With the mixer on low speed, blend in the chocolate mixture. Transfer to a larger mixing bowl.
Measure 1 cup of egg whites and freeze or discard the rest. Combine the cup of egg whites with a pinch of salt and 1 tablespoon of the remaining sugar in the bowl of an electric mixer fitted with the whisk attachment. Whisk on high speed until stiff but not dry. Mix half of the egg whites into the chocolate mixture; then fold the rest in carefully with a rubber spatula.
In the same bowl of the electric mixer fitted with the whisk attachment, whisk the heavy cream and the remaining 1 tablespoon of sugar until firm. Carefully fold the whipped cream into the chocolate mixture. Pour the mousse into a 2-quart serving dish. Cover with plastic wrap and chill for a few hours or overnight and up to a week.
Decorate with sweetened whipped cream just before serving.