Step 1

Separate the eggs whites from the yolks. Set the yolks aside, then put the egg whites in the bowl of a stand mixer fitted with a whisk attachment and whip on medium speed. Slowly add the granulated sugar to the egg whites in the mixer as they are whipping. Whip until soft peaks form.

Step 2

Over a double boiler, melt the chocolate and butter together. Remove from the heat and stir occasionally to help cool; let cool 10 minutes. One by one, add the egg yolks to the chocolate-butter mixture until fully incorporated. (The mixture will stiffen slightly.) Mix the whipped egg whites into the chocolate mixture in thirds, folding very gently so as to not mix the air out of the mousse.

Step 3

Carefully and evenly divide the mixture among 4 rocks glasses. Set the glasses in the refrigerator until fully chilled, about 30 minutes.

Step 4

For the chocolate liqueur whipped cream: Combine the heavy cream, liqueur and confectioners' sugar in the bowl of a stand mixer fitted with a whisk attachment. Whip on medium speed until soft peaks form.

Step 5

Spoon a dollop of the whipped cream into each of the chilled mousse glasses. Garnish each with a small sprig of mint.