Melt the chocolate in a bowl over hot water or the top of a double boiler. Make sure you don't get even a drop of water in the chocolate. Stir frequently with a spatula while doing the following steps and remove it from the hot water just as soon as it's melted.
Melt the butter. Mix together the powdered sugar, the melted butter and the vanilla. Don't bother sifting the powdered sugar. Just stir everything together with a wooden spoon until it's smooth and creamy. Stir in the peanut butter. This will break most flimsy wooden spoons if you're not careful. Mushing everything together with your hands works well and is lots of fun.
Press the fudge into a buttered 8 or 9 square pan. You don't need much butter, just wipe the butter papers over the inside of the pan. You can also line the pan with foil and butter that; this works especially well if you're making several batches in a row to give to people as holiday presents. Press the fudge in firmly; you don't want any air bubbles in it. You will get your hands messy here.
Pour the chocolate over the fudge and spread it in an even layer. The easiest way to do this is to shake the pan gently until the chocolate is even. Let the chocolate cool to room temperature. DO NOT PUT IT IN THE REFRIGERATOR.
Using a sharp knife cut the fudge into 1 inch squares. If you defied my instructions and put it in the refrigerator, the chocolate layer will shatter when you try to cut it. Unless you are going to serve it immediately, store it in the refrigerator. Let it come to room temperature before serving.
Notes: An extremely rich candy. The results resemble Reese's Peanut butter Cups that have attained Nirvana. Don't scrimp on the peanut butter; get the best you can find or make it yourself. If there's anything besides peanuts and salt in it, find another brand.
Unless you use the foil method, you will inevitably destroy at least one piece getting it out of the pan. The best method is to line the pan with foil, let the fudge cool, lift it out, peel off the foil, put it back in the pan and cut it. This keeps it from sticking to the pan. If you try to cut it out of the pan, it tends to fall apart.