Grease 9 inch spring form pan.
melt chopped chocolate with butter in double boiler.
Whisk together egg yolks and vanilla. Gradually stir in chocolate mixture till well blended.
Beat egg whites until soft peaks form. Gradually add sugar and beat until stiff peaks form. Fold 1/3 egg whites into mixture till blended. Gently fold in rest of egg white mixture.

Spoon batter into prepared pan spreading evenly

Bake at 370 deg F for 20 mins (do not over bake)