Make: 35 servings at 45¢ each. Prep: 20 minutes. Bake: at 350 F for 20 minutes. Decorate: 30 minutes.
Heat oven to 350 degrees F. Grease two 15 1/2 x 10 1/2 x 1-inch jelly-roll pans. Line bottoms of pans with waxed paper; grease and flour paper.
Stir together flour, baking powder and salt in bowl.
Beat eggs in large bowl with electric mixer until thick and lemon-colored. Gradually beat in sugar, lemon rind, lemon juice and vanilla. Alternately stir in flour mixture and cream, beginning and ending with flour mixture. Divide mixture evenly into prepared pans; smooth tops.
Bake in 350 degrees F oven for 20 minutes or until wooden pick inserted in centers comes out clean. Cool in pans on wire racks 10 minutes. Place wire cooling rack over each cake. Carefully invert cakes and racks. Remove pans from cakes; then carefully remove waxed paper. Handle cakes very carefully since they are thin layers. Cool to room temperature.
Spread jam in thin layer over top of one cake. Place another layer on top; place entire cake on tray.
Combine sugar and water in small heavy saucepan. Bring to boiling without stirring. Cover and boil for 2 minutes or until no sugar crystals remain. Uncover pan; continue boiling until syrup reaches soft-ball stage (syrup will form a rapidly flattening ball when dribbled into cold water); syrup will register 240 degrees F on candy thermometer.
While sugar syrup is cooking, beat egg whites, cream of tartar and salt in medium-size bowl with mixer until stiff, but not dry, peaks form.
At moment syrup reaches 240 degrees F or soft-ball stage, pour syrup into egg whites in fine, thin stream, beating constantly with mixer. When all the syrup has been added, continue to beat the egg whites at high speed until cool, for about 5 minutes.
In separate small bowl, beat butter with mixer until fluffy. Beat butter into cooled egg white mixture, 1 tablespoon at a time, until frosting is thick and creamy. Beat in vanilla. Use frosting immediately, or cover and store in cool place for 1 to 2 hours. (Do not refrigerate.) If frosting is too soft and thin, stir briefly over bowl of ice water to stiffen.
Frost the top and sides of the cake, saving some frosting for decorating. Spoon the reserved frosting into a pastry bag fitted with a decorative tip. Pipe a design on the sides of the cake. Pipe a fancy border around the top and bottom edges of the cake.