To make cake
Preheat oven to 350. Spray a 9 inche cake pan with non stick spray, line the pan with parchment paper and spray paper with the non stick spray.
Melt the chocolate in a microwave safe bowl carefully just until melted, set aside while preparing eggs.
Beat the egg whites and baaking powder in a large bowl to stiff peaks. Now beat in a different bowl the egg yolks, sour cream, sugar, salt and vanilla, beat well until thick and pale 3 to 4 minutes. Beat in melted chocolate until well blended.
Add egg white mixture in 3 additions to yolk chocolate mixture folding gently but combining well do not deflate
Pour intorepared pan. Bake on a cookie sheet for 30 to 45 minutes it should spring back when touched, do not overcook but be sure middle is cooked through. Cool on a rack before turning out on serving dish.
For optional glaze
In a small saucepan heat cream to a simmer, remove from heat and add chocolate, let sit 1 minute then stir until smooth, spoon over cooled cake allow to set
Whipped cream and berries are wonderful with this cake!