I learned how to make these peppermint-pattie topped brownies from a friend of mine. They're really easy to make once you get the timing right, but you have to know your oven. All you do is make a box of brownie mix (or make them from scratch if you like), and then take the pan out of the oven five minutes before they're done and put peppermint patties on top (I used 12 small ones for a 9x13" pan). Pop them back in for the last 5 minutes, and then smear the mints around as soon as they come out. It should create a nice marbled look.
The problem is, my oven is insane and cooks things at least five minutes faster than it's supposed to, even if I turn the temperature down a bit. It's very hard not to overcook things in it, but if you have a normal, well-adjusted oven, you should be fine. If you find that the brownies are already done when you take them out to put the peppermint patties on, don't bother trying to make this batch mint. I've tried melting the patties in the microwave, but then the melty stuff is just brown and not swirled and hardens way faster than you'd imagine. At that point, it's time to give up and start over.