Cut the cake rounds from a sheet cake made from a packaged mix. The tart lemony filling also starts with a mix and has a velvety smooth texture.
Heat oven to 350 degrees F. Coat 15 x 10 x 1-inch baking pan with cooking spray. Line with waxed paper; coat paper with cooking spray. Grate rind from lemon; squeeze juice (you should have 1/4 to 1/3 cup). Add water to juice to measure 1-1/3 cups.
In bowl, beat cake mix, egg whites, rind, juice-water mixture and extract to blend, 3 minutes. Fold in poppy seeds. Spread in prepared pan.
Bake in 350 degree F oven until cake springs back when pressed, 20 minutes. Let cool in pan on rack 10 minutes. Invert cake onto rack; remove pan and paper. Let cool.
Place milk in bowl. Sprinkle topping mix, pudding mix and extract over top. Beat at medium-high until thick and smooth, 4 minutes. If desired, spoon filling into pastry bag fitted with large star tip.
Using 2-1/4-inch round cutter, cut 24 rounds from cake. Snack on scraps.
Place 12 rounds on serving platter. Pipe or spread 2 level tablespoons filling onto each round. Top with remaining rounds and 1 slightly rounded tablespoon filling. Serve, or refrigerate up to 24 hours. Makes 12 servings.