COMBINE chocolate and butter 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.
FOLD egg whites into chocolate mixture until thoroughly blended. Pour into a greased 9-inch springform pan that has been lightly dusted with corn starch.
BAKE in preheated 300°F oven 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.