This recipe makes plenty for a 9 or 10 inch double layer cake.
It is a mousse-like frosting. You can add more confectioner's sugar at the end if you like it stiffer, or cream instead of milk if you want it even more like mousse. You can substitute vanilla or Grand Marnier for the Kahlua.

Melt 4 oz. unsweetened chocolate, cool and set aside.

In Mixer, beat 1+2/3 sticks unsalted butter.

Add gradually:

1 lb. confectioner's sugar
the cooled chocolate

You may need to add a little milk to keep it from collecting on the beater.

Add and beat well on high:

1/4 cup milk
1T. Kahlua or vanilla