In a microwave-safe bowl microwave butter and peanut butter with plastic wrap covering the bowl (punch small holes into the plastic wrap to let steam out) for 2 minutes on high. Stir and microwave on high for 2 more minutes. Add vanilla and powdered sugar to peanut butter mixture and stir to combine with a wooden spoon. Pour into a buttered 8 by 8-inch pan lined with wax paper. Place a second piece of waxed paper on the surface of the fudge and refrigerate until cool. Cut into 1-inch pieces and store in an airtight container for up to a week.