Place egg whites in a large bowl; let stand at room temperature for 30 minutes. In another large bowl, beat egg yolks until slightly thickened. Gradually add 1/3 cup sugar, beating until thick and lemon-colored. Blend in orange juice and peel. Add flour and remaining sugar to yolk mixture; mix well.
Add cream of tartar and salt to egg whites; beat with clean beaters until stiff peaks form. Fold into batter. Gently spoon into an ungreased 10-in. tube pan. Cut through batter with a knife to remove air pockets.
Bake at 325° on lowest oven rack 40-50 minutes or until top springs back when lightly touched. Immediately invert pan; cool completely, about 1-1/2 hours. Run a knife around sides and center tube of pan; remove cake.
For filling, in a double boiler or a metal bowl over simmering water, combine the sugar, flour and salt. Add the orange juice, egg and orange peel. Constantly whisk until mixture reaches 160° or is thick enough to coat the back of a spoon. Remove from heat and cool completely. Fold in whipped topping.
Cut cake horizontally into three layers. Place bottom layer on a serving plate; top with one-third of the filling. Repeat layers twice. Serve immediately or refrigerate. Yield: 12 servings.