Bake and cool cake as directed on package for two 23 cm round pans. Reserve one of the layers for another use. Freeze remaining layer about 2 hours or until firm. Cover cookie sheet with aluminum foil. Cut frozen layer horizontally in half. Place bottom half on cookie sheet. Spread with half of the yogurt. Top with remaining cake layer. Spread with remaining yogurt. Freeze about 4 hours or until firm. Heat oven to 230ºC F. Mix sugar and cocoa. Beat egg whites and cream of tartar in medium bowl with electric mixer on high speed until foamy. Beat in sugar mixture, 1 tablespoon at a time; continue beating until stiff and glossy. Spread meringue over top and side of cake, sealing completely. Bake about 5 minutes or until firm to the touch. Serve immediately, or return to freezer about 2 hours and serve frozen.