In a medium saucepan, whisk the sugar and eggs together over low heat. Cook, whisking frequently, until mixture reaches 75 C. Remove from heat.
In a small metal bowl over a bowl of simmering water, melt the semisweet chocolate, stirring often.
Whisk the melted chocolate and vanilla into the sugar and egg mixture. Set aside and let cool slightly.
Meanwhile, in a large bowl, beat the butter until light and fluffy. Beat in the chocolate mixture. Beat at high speed for 4-5 minutes or until light and fluffy. The colour of the mixture should lighten. Set aside.
In another large bowl, whip the heavy cream until soft peaks form. Add the icing sugar and beat until stiff.
Fold the whipped cream into the chocolate mixture until blended.
Spoon the chocolate mousse into glass bowls, ramekins, or other serving dish. You can spoon the mousse into separate bowls or into one large bowl. Chill the mixture for 4-6 hours. When ready to serve, make the topping.