In a large mixing bowl, combine cake mix, water, peanut butter, oil and eggs.
Beat on low for 30 seconds, and then on medium for an addition 2 minutes.
Fill paper cups half-full. Place a Reese's peanut butter cup in the center of each batter-filled cup. Top with an additional 1 Tbsp. batter, making sure to cover the entire peanut butter cup.
Bake at 350 degrees Fahrenheit for 18-22 minutes.
Cool in pan for 10 minutes before removing from muffin tin to wire cooling rack, where they'll cool completely before frosting.