Combine egg whites and cream of tartar in medium mixing bowl. Beat on medium speed until soft peaks form. Gradually sprinkle in remaining sugar and beat on high speed until stiff but not dry. Fold about a quarter of the egg whites into chocolate mixture to lighten it. Fold in remaining egg whites.
Scrape batter into prepared pan and level top if necessary.
Bake in preheated oven 30 minutes or until toothpick or wooden skewer inserted into center comes out with a few moist crumbs clinging to it. Cool in pan on wire rack. Torte will sink like a souffle. Taking care not to break edges of torte, slide knife between torte and pan and run it around pan to release cake completely. Invert cake onto plate and remove pan and paper liner.