Place egg whites in a large mixing bowl; let stand at room temperature for 30 minutes. Line a 15x10x1-inch baking pan with waxed paper; set aside.

Sift powdered sugar, flour, and 1/4 cup cocoa powder together 3 times. Beat egg whites and cream of tartar with an electric mixer on medium to high speed until soft peaks form (tips curl). Gradually add granulated sugar, about 2 tablespoons at a time, beating until soft peaks form (tips stand straight). Sift about one-fourth of the flour mixture over beaten egg whites; fold in gently. Repeat sifting and folding in the remaining flour mixture three more times, using one-fourth of the flour mixture each time. Spread batter in prepared pan.

Bake in a 375 degree F oven for 23 to 25 minutes or until top springs back when lightly touched. Loosen edges. Immediately invert cake onto a large wire rack; remove pan. Carefully peel off waxed paper. Cool thoroughly.

For filling, in bowl dissolve coffee granules in milk; add topping mix. Beat with an electric mixer on low speed until combined. Then beat on high speed until mixture form stiff peaks (4 to 5 minutes). Fold in yogurt.

To assemble, trim edges from cake. Cut cake crosswise into thirds. Place one cake layer on serving platter; spread with about one-third of the filling. Repeat two more times. Cover and refrigerate at least 2 hours or until serving time. Garnish top with chocolate curls or sift cocoa powder over cake, if desired. Makes 10 servings.