I love this cake and recipe from Amy at Milk Breath and Margaritas. It's a great way to celebrate the 4th of July.
Zest and juice of 2 lemons (use fresh lemons - it makes a difference)
2 boxes of white cake mix (I prefer Duncan Hines) (I was not paid to say that.)
6 large egg whites
1 t. lemon extract
1 C. (2 sticks) unsalted butter, room temperature
1 C. solid vegetable shortening, room temperature
4 boxes (1 pound each) confectioners' sugar
8 T. milk
4 T. lemon juice
(This recipe is already doubled to frost a large cake; you could half the frosting recipe)
3/4 C. blueberries, washed and dried
3 C. fresh large raspberries, washed and dried
1. Cake: Heat oven to 350. Coat two 15 x 10-inch 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 pans. Smooth tops.
2a. Let the helpers get a few licks in.
3. Bake at 350 until cake springs back when lightly touched, about 20 minutes. Let cool on wire racks 10 minutes. Invert cakes onto trays and 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 of frosting for piping. Frost layer, top with second layer, frost top and sides.
6. Mark off field for "stars" with toothpicks (approx. 6" across and 4.5" down) and fill in with blueberries. Place seven red "stripes" of raspberries on cake, leaving equal spacing for 6 white "stripes" between the rows of raspberries.
7. Place reserved frosting in pastry bag with star tip. Outline cake with piped stars, and fill in white stripes with piped stars.
8. Refrigerate uncovered to set frosting, then loosely cover with foil and refrigerate until ready to serve.