Sift 1 lb powdered sugar and cocoa in a 9×9 microwave safe dish.
Drop in 1/2 cup butter cut into pieces.
Pour in 1/4 cup milk.
Microwave on high for 2 to 3 minutes or until bubbly.
Stir in 1/2 cup chopped nuts, vanilla and peanut butter.
Refrigerate for one hour, then cut into squares.
Notes
Be sure to sift the powdered sugar and cocoa. Do not skip this step! It makes the fudge creamier. I have no idea why, but I've tried it both ways and sifting is so much better.