Break chocolate into small pieces and place in a heatproof bowl set over, not in, a pan of simmering water. Heat until melted and set aside.
In a heatproof bowl, set over, not in a pan of simmering water, beat the egg yolks with 2 tablespoons sugar iuntil thick, add the Grand Marnier.
Beat the egg whites until soft peaks form, gradually add the remaining sugar and beat until stiff. Beat the cream until stiff.
Fold the melted chocolate into the egg mixture.
Gently fold the cream and egg whites into the chocolate mixture.
Spoon the mousse into a large serving bowl, cover with plastic wrap and refrigerate until set.