Place the chocolate and butter in a heatproof bowl and place over a saucepan containing barely simmering water (or use a double boiler). Melt the chocolate and butter together and stir with a wooden spoon until smooth. Remove from heat and cool slightly. Add the egg yolks to the chocolate, 1 by 1, beating with a whisk until incorporated. Set aside.

In another bowl, beat the egg whites until foamy. Add the cream of tartar and continue to beat. Gradually whisk in 1/4 cup sugar and continue beating until stiff peaks form.

Beat heavy cream in a chilled bowl until it begins to foam and thicken up. Add the remaining 2 tablespoons sugar and vanilla. Continue to whip the cream until it holds soft peaks.

Gradually and gently fold the egg whites into the chocolate mixture to lighten it. Then, delicately fold in the whipped cream. Take care not over work the mousse. Divide mousse into 4 individual glasses. Cover and chill for several hours. Garnish with whipped cream and chocolate shavings before serving.