Place butter and peanut butter in a large microwave safe bowl. Place in the microwave and cook for 2 minutes. Remove and stir, place back in microwave and cook for about another minute depending on your microwave. It gets super hot so handle with care. If you're microwave is not powerful you may need to go an additional minute.
Place on counter, whisk the butter and peanut butter until smooth. Add the vanilla, then the powdered sugar. Stir with a wooden spoon until it comes together and loses it gloss.
Transfer to a parchment lined 8 by 8 inch pan. Make sure you leave some parchment hanging over the edges so you can pull it down to cover the fudge while it sets, and also it makes it easier to remove to cut.
Smooth the fudge inside the pan. Fold over the parchment so the fudge is covered and not exposed to air. Pop in the fridge till it sets up. Takes a couple of hours.
Remove from fridge and set on counter in pan.
In a microwave safe bowl, melt the chocolate in 30 second increments until it's melted and smooth. Spoon evenly over fudge.
Toss the chopped up pieces of peanut butter cups over top on the warm chocolate so they stick.
Pop back in the fridge for a couple of hours.
Serve with a big old peanut butter smile!