Preheat the oven to 350 degrees F. Combine peanut butter, sugar and eggs in the bowl of a stand mixer. Beat until well combined. Divide the dough evenly between 24 wells of a mini-muffin pan (I usually have a bit left over, but do so to leave room for the candy). Make an indentation in the center of each dough ball. Bake for 13-15 minutes, until almost done. Remove from the oven and press a peanut butter cup into the center of each cookie until it is flush with the top of the dough. Top each cookie with 3-4 mini marshmallows. Return the pan to the oven and bake 2-3 minutes more, until the marshmallows have puffed up. Transfer the pans to a wire cooling rack and allow the cookies to cool completely in the pan before removing them.
Directions for the Ganache:
Put the chocolate in a small deep heatproof bowl. Bring the heavy cream to a full boil, then pour it over the chocolate. Wait for 1 minute, then gently whisk the cream into the chocolate: start at the centre and slowly work your way out in concentric circles until you have a smooth, shiny mixture. Gently whisk in the butter.
Line a small baking sheet with wax paper. One by one, hold a madeleine at its narrow end and dip it into the chocolate, then lift it up, let the excess chocolate drip back into the bowl and place smooth side down on the wax paper. Slide the baking sheet into the refrigerator to set the glaze, about 15 minutes (You’ll have more ganache than you need, but making a larger quantity produces a better ganache. The leftover dip can be covered and refrigerated for 1 week or frozen for up to 1 month.)
If you’d like, pipe a little squiggle of fluff on the top of each madeleine once the chocolate is set.