6 egg whites
1/2 tsp. finely shredded lemon peel
1 tbsp. lemon juice
1/2 tsp. vanilla
1/4 c. sugar
4 egg yolks
1 tbsp. all-purpose flour
Confectioners' sugar (optional)

Generously butter a 12 x 7 1/2 x 2-inch baking dish or 2-quart baking dish; set aside.

In a large mixer bowl, combine egg whites, lemon peel, lemon juice and vanilla; beat with mixer on medium speed until soft peaks form (tips curl). Gradually add sugar, beating at high speed until stiff peaks form (tips stand straight).

In small mixer bowl, beat the egg yolks with mixer on high speed about 5 minutes, or until thick and lemon-colored. Fold in flour. Fold some of the beaten egg white mixture into yolks. Gently fold egg yolk mixture into remaining stiff-beaten egg whites.