White Batter (Basically Angel Food):
1 3/4 cup egg whites
1 1/4 cups sifted cake flour (sifted before measuring)
1 1/2 cups sugar
1/2 tsp salt
1 1/2 tsps cream of tartar
1 1/2 tsps vanilla extract
Place egg whites in a large bowl for 1 hour so they can come to room temperature.
Sift cake flour with 1/2 cup of the sugar; resift 3 times.
Using an electric mixer at high speed, beat egg whites with the salt and cream of tartar until soft peaks form when beater is slowly raised. Beat in the remaining sugar, 1/4 cup at a time, beating well after each addition. Beat mixture until stiff peaks form when beater is slowly raised.
Using a wire whisk, fold the vanilla extract into the egg whites until combined.
Sift the flour mixture, one fourth at a time, over the egg whites.
Using the wire whisk with an over and under motion, gently fold in each addition with 15 strokes, rotating bowl a quarter of a turn after each addition. Fold ten more strokes to blend completely. Turn 1/3 of the batter into a medium bowl.
Preheat oven to 375 degrees.
Make the Yellow Batter:
5 egg yolks
2 tbsps cake flour
2 tbsps sugar
2 tbsps grated lemon peel
In a small bowl combine the egg yolks, flour, and sugar. With mixer on high speed, beat until very thick; add the lemon peel.
With a wire whisk, using the over and under motion, fold the yolk mixture into the reserved one third batter with 15 strokes.
For marbling, spoon the batters alternately into an ungreased 10-inch tube pan, ending with white batter on top. With knife, cut through batter twice. With rubber scraper, gently spread batter in pan until it is smooth on top and touches side of pan all around.
Bake on the lower oven rack 35 to 40 minutes or until the cake springs back when pressed with fingertip. Invert pan over neck of bottle to cool 2 hours. With a spatula, loosen cake from the pan; remove. Sprinkle with powdered sugar if desired.