Spray bottom and side of 9-inch springform pan with cooking spray; dust side with 2 tsp. sugar.
Beat egg whites in small bowl with mixer on high speed until stiff peaks form; set aside. Microwave semi-sweet chocolate and butter in large microwaveable bowl on HIGH 1-1/2 min. or until chocolate is completely melted and mixture is well blended, stirring every 45 sec. Stir in 1/2 cup of the remaining sugar. Add egg yolks, 1 at a time, stirring after each until blended. Stir in sour cream.
Add egg whites to chocolate mixture in batches, gently stirring after each until blended. Stir in nuts. Pour into prepared pan.
Bake 35 min. or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Run knife around rim of pan to loosen cake; cool before removing rim.
Melt white chocolate as directed on package. Beat cream cheese and remaining sugar in large bowl with mixer until blended. Add white chocolate; mix well. Gently stir in COOL WHIP; spread over cake. Refrigerate 4 hours.