Place chocolate in a medium metal bowl. Set bowl over saucepan of simmering water; stir until melted and smooth. Stir in butter until melted. Set aside.
In another large bowl, with the mixer on high speed, beat egg whites and sugar until soft peaks form.
Gently fold whites into chocolate mixture. Next, fold whipped cream into whites-chocolate mixture until fully incorporated. Spoon into 8 dessert cups. Refrigerate at least two hours before serving.
Sprinkle with coarse sugar to give texture to the smooth mousse.