Microwave butter and peanut butter for 2 mins. on high. Add vanilla and powdered sugar to peanut butter mixture and stir to combine with a wooden spoon, Pour into a butter 8x8 inch pan lined with wax paper. Place a second piece of wax paper on the surface of the fudge and refrigerate until cool. Cut into 1 inch pieces and store in an air tight container for up to one week.