PREHEAT oven to 350°F. Grease and flour 9-inch springform pan. Beat egg yolks and 1/2 cup of the sugar in medium bowl with electric mixer on high speed until thick and lemon colored. Add butter; mix well. Blend in chocolate and orange peel. Stir in walnuts; set aside.

BEAT egg whites in small bowl with electric mixer on high speed until foamy. Gradually add remaining 1/4 cup sugar, beating until stiff peaks form. Add to chocolate mixture; stir gently until well blended. Spread into prepared pan.

BAKE 35 to 40 min. or until wooden toothpick inserted in center comes out clean. Cool in pan on wire rack 10 min. Run a knife or metal spatula around rim of pan to loosen; remove rim of pan. Cool completely on wire rack.