Preheat oven to 350°. Line two 12-cup muffin pans with cupcake liners; set aside.
Beat cake mix, Hellmann’s® or Best Foods® Real Mayonnaise, water and eggs in large bowl with electric mixer on low speed 30 seconds. Beat on medium speed, scraping sides occasionally, 2 minutes.Scoop or spoon into prepared muffin cups.
Place one Reese’s peanut butter cup into the center of each cupcake. Press the peanut butter cup into the batter so 2/3 of the peanut butter cup is covered with batter. The peanut butter cup will sink further in the cupcakes as it cooks.
Bake 18 minutes or until toothpick inserted in centers comes out clean. Cool 10 minutes on wire racks; remove from pans and cool completely.
Place the peanut butter and butter into a large bowl. Beat with a mixer until well blended
Add half the sugar and half the milk to the butter mixture. Mix until blended.
Add the remaining sugar and milk. Mix until blended and the beat with mixer for 3 minutes until nice and fluffy. Add more milk if needed to received desired consistency.
Pipe frosting onto cooled cupcakes. Cut remaining peanut butter cups in half. Place 1 half on top of each cupcakes.