Use double-boiler over low heat to melt the butter and chocolate together. Just before chocolate finishes melting, remove it from the stove and beat with a wooden spoon until smooth.
Transfer the chocolate mixture to a bowl and beat in the egg yolks with a wisk. Refrigerate.
Beat the egg whites with half the sugar until stiff peaks. Set aside. Beat the cream with the remaining sugar and vanilla until soft peaks.
Stir a bit of the whites into the chocolate mixture to lighten it, then fold in the remaining whites gently. Fold in the cream an mix well. Divide the mousse among six cups. Refrigerate until chilled.