Zest and juice of 2 lemons
2 boxes (18.25 oz. each) white cake mix
6 large egg whites
1 tsp. lemon extract
1/2 cup (1 stick) unsalted butter, at room temperature
1/2 cup solid vegetable shortening, at room temperature
2 boxes (1# each) confectioners' sugar
4 T. milk
2 T. lemon juice
3/4 cup fresh blueberries, washed and dried
3 cups fresh large raspberries, washed and dried
1. Cake: Heat oven to 350º. Coat two 15 x 10" jelly-roll pans with nonstick cooking spray. Pour lemon juice into 4-cup measuring cup; add enough water to equal 2-2/3 cups.
2. In large bowl, beat cake mixes, egg whites, lemon zest, juice-water mixture, and lemon extract to blend, about 3 minutes. Divide evenly between two prepared pans; smooth tops.
3. Bake at 350º until cake springs back when lightly touched, about 20 minutes. Let cool on wire racks 10 minutes. Place rack over each cake. Carefully invert cakes and racks. Carefully remove pans from cakes and let cool to room temperature. If making ahead, wrap layers when cool in plastic wrap; refrigerate for several days. Bring to room temperature before proceeding.
4. Frosting: Beat butter and shortening in large bowl 1 to 2 minutes until fluffy. Add confectioners' sugar, milk, and lemon juice. Continue beating on low until well blended and good spreading consistency, adding more milk if needed.
5. Decoration: Place one layer on serving tray. Reserve 1 cup frosting for piping. Frost layer with 1 cup frosting; top with second layer; frost top and sides. Place reserved frosting in pastry bag with a star tip. Outline cake with piped stars.
6. Mark off a field for "stars" with toothpicks - 6" across and 4-1/2" down. Fill in with blueberries. 7. Place seven red "stripes" of raspberries on cake, leaving equal sapcing for 6 white "stripes" between them. Carefully pipe white frosting "stripes" between the rows of raspberries. Refrigerate uncovered to set frosting, then loosely cover with nonstick aluminum foil. Refrigerate until ready to serve. Let stand at room temperature for 20 minutes before serving.