In a mixing bowl, mix apple sauce and splenda. Beat in eggwhites, one at a time.
Add lemon peel, lemon juice, and vanilla; mix well.
Combine dry ingredients; add to apple sauce mixture alternately with sour cream (batter will be thick).
Fill greased or paper-lined muffin cups with 1/4 cup of batter. Bake at 350 degrees F for 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes; remove to wire racks to cool completely.
For frosting I like to mix maple syrup with some lemon juice and whiskey, and soak bottom of the cup cakes in it, or just use some sugar-free coolwhip.