HEAT oven to 350ºF.
BEAT egg whites in small bowl with mixer on high speed until foamy. Gradually beat in 1/4 cup granulated sugar until soft peaks form.
BEAT butter and remaining granulated sugar in large bowl with mixer on medium speed until light and fluffy. Add chocolate and egg yolks; mix well. Stir in combined nuts and flour. Add egg whites in batches, gently stirring after each addition just until blended.
POUR into 9-inch springform pan sprayed with cooking spray.
BAKE 40 min. or until toothpick inserted in center comes out clean. Cool 10 min. Run knife around rim of pan to loosen dessert; remove rim. Cool dessert completely. Sprinkle with powdered sugar before serving.