Tiramisu

Prepare Sponge Cake: Heat oven to 400 degrees F. Grease a 15 1/2- by 10 1/2-inch jelly-roll pan; line bottom with waxed paper; grease and lightly flour waxed paper.

In medium-size bowl, sift together flour, baking powder, and salt; set aside. In small bowl, with electric mixer on medium speed, beat egg yolks and 3/4 cup sugar until very pale and thick -- about 5 minutes. Add lemon juice, water, and vanilla; beat until well combined, scraping side of bowl once during beating. Set aside yolk mixture; wash and dry beaters.

In large bowl, with electric mixer on low speed, beat egg whites 1 minute or until frothy. Increase the speed to medium and gradually add remaining 1/4 cup sugar, beating until whites appear glossy and stiff but not dry -- 2 to 3 minutes.

Sift half the flour mixture over the egg-yolk mixture and add half the beaten egg whites; with rubber spatula, fold until combined. Fold yolk mixture, remaining flour mixture, and the butter into remaining egg whites, just until combined. Gently spread batter evenly into prepared pan.

Bake 15 to 20 minutes or until top is golden brown and center springs back when gently pressed. Cool cake in pan on wire rack 15 minutes; invert cake onto rack, remove waxed paper and cool completely. (Sponge Cake can be made ahead, cooled, wrapped in plastic, and refrigerated for use within 2 days or frozen for use within several months.)

When ready to assemble, prepare Filling: In small bowl, with electric mixer on medium speed, beat heavy cream until soft peaks form; set aside. In large bowl, with electric mixer on low speed (no need to wash beaters), beat mascarpone and 1/2 cup sugar just until blended. Fold whipped cream into mascarpone mixture just until smooth and blended. In small bowl or glass measuring cup, combine remaining 1/4 cup sugar and the coffee.

To assemble Tiramisu, cut half of Sponge Cake to make a round piece to line bottom of 2-quart serving dish. Brush half of coffee mixture and half of sherry over bottom cake layer. Top with 1 tablespoon sifted cocoa and half the chocolate. Spread half of mascarpone mixture evenly over bottom layer. Cut remaining cake to fit dish and cover mascarpone layer. (Leftover cake trimmings can be reserved for another use.) Brush top cake layer with remaining coffee mixture and sherry; top with 1 tablespoon sifted cocoa and remaining chocolate. Spread remaining mascarpone mixture over second cake layer. Sift remaining 1/2 tablespoon cocoa over top and refrigerate Tiramisu until ready to serve.