5 egg yolks, room temp.
2/3 c. sugar
3 tbsp. each lemon and lime juice
3 tbsp. water
2 tbsp. potato starch
2 1/2 tsp. each, grated lemon and lime peels
1/4 c. (1/2 stick) non-dairy butter, cut into sm. pieces

2/3 c. sugar
2 tbsp. water
1 tbsp. fresh lemon, OR lime juice
1/2 tsp. vanilla
1 egg white
1 tsp. honey
Pinch of salt

4 eggs, separated, room temp.
4 tbsp. sugar
1/4 c. potato starch
Pinch of salt
1 tsp. grated lemon peel
1/2 c. matzo cake meal

Filling: Mix yolks, sugar, juice, water, potato starch, grated peel in small saucepan over medium-low heat. Add butter and boil until mixture is very thick and peels away from sides. Stir constantly for one minute. Transfer to bowl and cover with plastic wrap directly on surface to prevent skin from forming. (This can be prepared one day ahead and refrigerated.)

Frosting: Beat sugar, egg whites, water, lemon juice, honey, salt in bowl for one minute. Set bowl over saucepan of briskly simmering water (do not let water touch bowl). Beat at high speed until stiff peaks form, and is spreadable (about four minutes). Remove bowl and continue beating until cool and texture of marshmallow fluff (about 3 minutes). Add vanilla. Cover and cool in refrigerator (can be prepared one day ahead).

Cake: Preheat oven to 400 degrees. Grease bottom of 15 1/2 x 10 1/2 jelly roll pan. Line pan with waxed paper. Grease paper. Beat whites and salt in bowl until soft peaks form. Add sugar, 1 tbsp. at a time, and beat until stiff. Blend yolks and add 1 tbsp. lemon peel in bowl. Fold in 1/4 of whites. Fold mixture back into rest of egg whites. Sift potato starch and cake meal over and fold gently. Turn batter into prepared pan, spreading evenly. Bake until light brown, approximately 10 minutes. Run knife around side of cake and invert onto towel lined rack. Cool.

To assemble: Stir 1/3 cup of frosting into filling. Mix 2 tsp. lemon peel into remaining frosting. Discard paper from cake. Spread cake evenly with filling. Roll up tightly as for jelly roll, starting at short end, and using towel as aid. Transfer cake to platter, seam side down. Spread frosting over entire cake. (This can be assembled 8 hours ahead.) Tent loosely with foil and refrigerate. Bring to room temperature before serving.