Step 1

Grease an 8-inch square pan with butter. Trim a piece of parchment paper to 8 inches wide and line the pan with the parchment sling, leaving enough overhand on two sides to cover the top of the fudge with later.

Step 2

Combine the butter and peanut butter in a 4-quart microwave-safe bowl and cover with plastic wrap. Poke a few holes in the plastic wrap to vent the steam, then microwave for 2 minutes on high. Stir and microwave on high for 2 more minutes. (Use caution when removing this mixture from the microwave, it will be very hot.) Add the vanilla and powdered sugar to the peanut butter mixture and stir to combine with a wooden spoon. The mixture will become hard to stir and lose its sheen.

Step 3

Spread mixture into prepared pan. Fold the excess parchment paper so it covers the surface of the fudge and refrigerate until cool, about 2 hours. Cut into 1-inch pieces and store in an airtight container at room temperature for up to a week.