Just salted peanuts plus a tablespoon of oil (peanut oil, rape seed oil, whatever). Dump these in a blender and blend 'till the right consistency is achieved. You'll probably have to help the blender a bit by stirring in the peanuts when the knives have created a hole in which they are spinning freely, repeat this until the mixture has gotten warm enough to start flowing by itself. Make sure not to touch the knives while doing this and use a wooden utensil to avoid damage to the knives. Plastic works as well but just in case you happen to touch the knives I'd advice using wood or you'll end up eating plastic.

A full jar of peanut butter takes about 1.5 to 1.8 jars of peanuts (and a tablespoon of oil) to produce.