Place egg whites in a large bowl; let stand at room temperature for 30 minutes. Sift 3/4 cup sugar, cornstarch, flours and potato starch together twice; set aside., Add cream of tartar, salt and vanilla to egg whites; beat on medium speed until soft peaks form. Gradually add remaining sugar, about 2 tablespoons at a time, beating on high until stiff peaks form. Gradually fold in flour mixture, about 1/2 cup at a time., Gently spoon into an ungreased 10-in. tube pan. Cut through the batter with a knife to remove air pockets. Bake on the lowest oven rack at 350° for 35-40 minutes or until lightly browned and entire top appears dry. Immediately invert pan; cool completely, about 1 hour., Run a knife around side and center tube of pan. Remove cake to a serving plate. If desired, dust with confectioners' sugar and serve with fruit.