A fairly simple task I thought - chocolate, peanut butter, sugar. Reese's list the ingredients as milk chocolate, peanuts, sugar, salt (& preservative).
The only other Instructable I could find like this is this one. The method of construction is different, but probably a bit easier if you don't want the classic cup-shape.
Thank you for the peanut butter cups, i have just eaten one, it was lush. X (verdict by SMS)
Step 1: Ingredients & other materials
~100g Icing-sugar, otherwise known as confectioner's sugar. This packet is white powdered refined cane-sugar.
~100g Peanut butter (smooth). This jar claims the ingredients to include 92% roasted peanuts and some salt. Yes the label does advise that it "Contains peanuts."
~200g Chocolate. Use what you like, I went for "Chocolate flavour cake covering", as this is the sort of thing my sister likes.
For making the cups, I used some "white baking cases", an improvised bain-marie and a few other common kitchen tools.