Line an 8 x 8 inch pan with parchment paper (I recommend making the parchment paper long enough so that it hangs off the edges of the pan on two sides, which will allow you to grip them later to lift out the parchment paper and contents.) 

Combine the butter and peanut butter in a microwave-safe bowl. Microwave for 1 minute and stir. Microwave for 1 more minute and stir again until mixture is completely melted and smooth. Add the powdered sugar to the peanut butter mixture and stir until batter is smooth. The batter should be quite thick, like a paste.

Spread batter into the lined baking pan. Press another parchment paper on the surface and use your palms to apply gentle pressure to evenly spread the fudge across the entire pan. Lift off the top layer of parchment paper and smooth surface of fudge with your hands. Refrigerate for about 2 hours to let set. Lift fudge out of the pan. Cut into one-inch squares with sharp knife. Keep uneaten portions stored in the fridge.