I got this recipe from emilybites and it's AMAZING!
1 (18.25 oz) box of yellow cake mix (I used Betty Crocker Super Moist)
1 can of diet lemon-lime soda (I meant to use Sprite Zero)
2 egg whites
8 oz Neufchatel cheese (also labeled 1/3 less fat cream cheese) - do not substitute fat free
1 c powdered sugar (I used 1/2 c splenda, 1/2 c powdered sugar)
1. Preheat oven per directions on cake mix box.
2. Line cupcake pan(s) with 24 cupcake liners or use cooking spray to lightly coat 24 spots
3. Zest both the lemon and lime and set zest aside. Cut both fruits in half. Using a reamer, or by hand, ream/squeeze the juice from half a lemon and half a lime into a large bowl. Set the other halves aside.
4. In the large bowl with the lemon & lime juice, add the cake mix, soda, egg whites and half of the lemon and lime zest. Beat together with an electric mixer until thoroughly mixed. Pour mixture evenly into cupcake spaces, filling each about 2/3 full.
5. Bake according to directions on cake mix box. Let cool completely before frosting.
6. In a separate bowl, squeeze or ream the juice from the remaining lemon and lime halves* and add the remaining zest, the Neufchatel cheese and the sugar. Beat with an electric mixer until smooth and then use to frost the cupcakes.
Yields 24 servings. WW P+: 3 per serving; 126 calories per serving.