Line an 8x8-inch pan with parchment paper, leaving an overhang to lift the fudge out later. Lightly grease the parchment for easy release.

In a microwave-safe bowl, combine the butter, milk, and peanut butter.

Microwave on high for 30–60 seconds. Remove and stir. If not fully melted and smooth, microwave in additional 15–20 second bursts, stirring after each, until uniform and hot but not boiling.

Remove the bowl from the microwave and add about half the confectioners’ sugar. Stir briskly to combine; it will be thick. Add the remaining sugar and stir until you have a smooth, spoonable mixture with no dry streaks.

Transfer the mixture to the prepared pan. Use an offset spatula or the back of a spoon to press it evenly into the pan and smooth the top.

Chill in the refrigerator until firm, 30–60 minutes. For quicker setting, place in the freezer for 15–20 minutes.

Lift the fudge from the pan using the parchment overhang and cut into squares.