Combine the butter and peanut butter in a 4-quart microwave-safe bowl and cover with plastic wrap. 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. Spread into a buttered 8 by 8-inch pan lined with parchment paper. 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.
Recipe courtesy of Alton Brown, 2003