Preheat oven to 350 degrees.
Mix cake mix, water, applesauce (or oil), and eggs according to directions on box (2 minutes on medium speed with electric mixer).
If desired, add chocolate chips to the cake batter.
Fill muffin pan with liners, and fill liners 2/3 full with batter. Bake according to directions on package, about 18-20 minutes.
Allow cupcakes to cool for about 10 minutes, then gently press one peanut butter cup into each cupcake.