Place chocolate and butter in a medium bowl. Set the bowl over a pan of simmering water stirring occasionally until melted. Once melted, remove from heat and set aside to cool.
Combine the yolks, sugar, liqueur of choice and vanilla extract in the bowl of an electric mixer, or a medium sized bowl. Place the bowl over a pot of simmering water and whisk until very warm and thickened. Once thickened, remove from heat, place in electric mixer and whip until cold and very thick. If you don’t have an electric mixer, just bust out a whip and give it some arm action. Set aside.
In a bowl of an electric mixer or using a hand mixer or hand whip, beat 1 cup of heavy cream until it holds stiff peaks. Set aside. Working quickly, add the melted chocolate to the thickened yolk mixture and fold thoroughly. Add the heavy cream to the yolk/chocolate mixture and fold quickly, but gently until no white streaks remain. Place in the freezer to chill for 15-20 minutes.
While the mousse is chilling, make the chocolate cups.