2Place egg whites in medium mixing bowl; set aside. With a large slotted spoon, beat yolks lightly. Add vanilla and sugar. Mix well.
3Sift cocoa through strainer and slowly add to yolk mixture. Cream together until smooth.
4Beat egg whites until peaks form, but not too stiff. Fold gently into cocoa mixture. pour into ungreased spring-form pan. Bake 35-40 minutes. Test with toothpick. Let cake set until center falls in and cake is completely cool. Remove band after first loosening along sides with a knife. Do NOT try to pull away from pan bottom. Remove the band only.
5Make the filling: Combine all ingredients and beat stiff. pole into center of cake. Chill at least 6 hours, or overnight.