You can make the brownies with ganache 3 days ahead, cover with plastic wrap (making sure it does not touch ganache), and chill. The tops can be made up to a week ahead and stored airtight at room temperature. Look for hard, round peppermint swirl candies, also called starlight mints, with bright red color; we liked Golden Gate Goodies brand (99 cents for a 6-oz. bag; available at Longs Drugs; visit longs.com for store locations). Keep an eye on the candies as they melt; some brands don't spread as quickly or as thinly as others. You may want to make a test batch using 12 mints arranged in 3 rows of 4 to determine whether you'll need to adjust your oven's temperature or space the candies closer together.