~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.
If you take baking cases out of the packet, you tend to find them sagging, add weight to them and they'll sag some more. So I cut up a kitchen-tissue inner-tube to hold the sides of baking-cases vertical, which did a pretty good job. Melt the chocolate in whichever way works for you. I improvised a bain-marie by sitting a bowl on top of a pan simmering water gently. Add a spoon of molten chocolate to a baking case, and push it up the sides with a small spoon. It will relax back down to the bottom, leaving a coating on the sides. Leave the chocolate to cool and move on to the next case.
To finish the cups, add a spoon of chocolate to the top, give a bit of a shake, and let them cool. With a bit of practice I could do better, and so could anyone else. But the things worked, I ate one it was just fine (and I don't really like these things). I made 10 cups from the ingredients, they'll be ~40g (twice the Reese's) ~200 kcal.: they are high in fat and sugar.