I found this recipe on Alton Brown's website. He is great to watch in the kitchen, if you've never seen him give him a try. He explains things so that even I can understand it.
1 cup butter
1 cup peanut butter
1 teaspoon vanilla
1 pound (16 ounces) powdered sugar
Butter a 8x8 dish.
Microwave butter and peanut butter for two minutes. Stir to combine. Add vanilla and powdered sugar to peanut butter mixture and stir with a spoon as much as you can. The mixture will get the consistency of sugar cookie dough. You will be able to pick it up all at once. Smooch it around with your hands if the powdered sugar still needs some mixing in.
Press the fudge into the dish using your hand or a spoon. Place plastic wrap over the top of the fudge & let cool in the fridge.