Melt together the chocolate and butter and cool slightly for 1-2 minutes.

Beat the egg yolks into the chocolate mixture with a whisk until smooth and set aside.

In a separate small bowl, using an electric mixer, beat the egg whites cream of tartar and sugar until until stiff peaks form. Set aside.

In another bowl pour in the heavy cream and vanilla and whip until it forms soft peaks, about 5-7 minutes.

Combine the chocolate, egg, and cream mixtures by gently folding being careful not to over mix.

Spoon into dessert cups, ramekins, or glasses and chill for several hours.