Move oven rack to lowest position. Preheat oven.

Sift together powdered sugar, flour and cocoa. Beat egg whites and cream of tartar in large bowl on medium speed until foamy. Beat in granulated sugar, 2 tablespoons at a time, on high speed, adding salt with the last addition of sugar. Continue beating until stiff and glossy. Do not under-beat. Sprinkle cocoa mixture, 1/4 cup at a time, over meringue, folding in just until cocoa mixture disappears. Spread batter into ungreased tube pan.

Gently cut through batter with metal spatula. Bake until cracks feel dry and top springs back when touched lightly. Invert pan onto metal funnel or glass bottle about 2 hours or until cake is completely cool. Remove from pan Slice off top of cake about 1 inch down; set aside. Cut down into cake 1 inch from outer edge and 1 inch from edge of hole, leaving substantial "walls" on each side. Remove cake within cuts with curved knife or spoon, being careful to leave a base of cake 1 inch thick. Spoon sherbet into cake cavity; smooth up. Replace top of cake. Cover and freeze about 3 hours or until firm.