Place the chocolate into a bowl and bain marie (over a pan of simmering water) and melt it, taking care not to let the chocolate over heat, you don't want it burning on side or getting too hot. Then cool.
Meanwhile, whisk the egg whites until they form soft peaks.
In a separate stir the liqueur into the beaten egg yolks and add to the melted chocolate.
Pour the chocolate mixture into the egg whites and gently fold together.
Divide evenly between four glasses or ramekins and chill.