1. 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.
2 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 inch tube pan. Cut through batter with a knife to remove air pockets.
3. Bake at 325 degrees on lowest oven rack for 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.
4. 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 degrees or is thick enough to coat the back of a spoon. Remove from heat and cool completely. Fold in whipped topping.
5. Cut cake horizontally into three layers. Place bottom layer on a serving plate; top with one-third filling. Repeat layers twice. Serve immediately or refrigerate. Yield: 12 servings.