Melt the chocolate over a low heat or in the microwave (stirring regularly). Allow it to cool a little then add the egg yolks. 

Whip the egg whites until white with stiff peaks. Whip the cream until it's good and stiff too. First - fold the cream into the Toblerone. Then add the agg whites and liqueur and fold that through with a wooden spoon or spatula. Spoon the mousse into 6 wee serving dishes and put it in the fridge until set. 

Tip: This is brilliant with berries and cream.