Preheat oven to 350 degrees F. Spray or grease a 13x9-inch oblong baking pan.
Combine cake mix, egg whites, evaporated milk, water and oil in a large bowl. Beat for 2 minutes at medium speed.
Turn batter into pan and bake as for 13x9 instructions on the box. (varies, I used Duncan Hines and it was 25-30 minutes, but check the box for the brand you are using).
Remove cake from oven and cool for 30 minutes. Using a large-tyned fork (I used a meat serving fork), prick holes in the cake about 1/2-inch apart.
Mix the gelatin in a small bowl with the boiling water, mixing until sugar and gelatin is dissolved. Add the 1/2 cup cold water and stir well. Pour gelatin over top of cake. Refrigerate for at least 4 hours, preferably overnight.
For frosting, beat the softened butter and the softened cream cheese until very well combined. Beat in the lemon zest.
Add the confectioners sugar, 1 cup at a time, beating until a thick paste texture is reached. Thin down to a spreadable consistency with the reserved lemon juice (I used about 2 tbs of lemon juice).
Frost chilled cake with the icing and garnish with lemon and lime slices if desired.
Makes 12 servings. Store leftover cake in the refrigerator.