Preheat oven to 300°. Spray 9-inch springform pan with cooking spray; dust lightly with additional corn starch.
Combine chocolate and margarine in small saucepan. Cook over low heat,
stirring frequently, just until chocolate melts. Pour into large bowl; cool to room temperature.
Beat egg whites until foamy in medium bowl with mixer at high speed. Gradually add sugar, beating until soft peaks form. Add egg yolks to chocolate mixture; stir until blended. Stir in corn starch.
Gently fold in egg whites until thoroughly blended. Pour into prepared pan. Bake 40 to 45 minutes or until set.
Run knife around edge immediately after removing from oven. Cool on wire rack. Remove side of pan. Sprinkle with powdered sugar.