In the top of a double boiler over simmering water, melt the chocolate, stirring until smooth. Remove it from the heat and whisk in the butter. Whisk in the yolks until well-blended. Set aside.
In a medium-bowl, use an electric mixer on medium-high speed, beat the egg whites until they begin to form medium peaks. Add 1 tablespoon of sugar and beat 30 seconds longer. Gently fold the egg whites into the chocolate mixture just until no white streaks remain.
Wash and dry the bowl and beaters. Add the cream and remaining ¼ cup sugar and beat at medium-high speed until medium peaks form. Mix in the vanilla extract, then fold into the chocolate mixture until no white streaks remain.
Divide the mousse between 6 glass serving dishes or small jars. Garnish with chocolate shavings. Cover and refrigerate for at least 2 hours or up to 2 days.