Melt chocolate in double broiler, remove from heat and cool slightly

Add egg yolks 1 by 1 into chocolate mixture with a whisk, then set aside

In another bowl, beat egg whites until foamy, then add cream of tarter and continue to beat. Gradually whisk 1/4 cup sugar into this and continue to whisk until stiff peaks form

Beat heavy cream into a chilled bowl until it begins to foam and thicken up. Add 2 tbsp of sugar and vanilla, continue to beat until soft peaks form

Gradually and gently fold the egg white into chocolate mixture to lighten it. Then delicately fold into whipped cream. Take care not to over work the mousse.

Cover and chill