Preheat oven to 350 degrees, and center an oven rack for baking.
Using about a tablespoon or two of room temp butter, grease a 9x13 baking dish, especially the corners. Line with a piece of parchment paper as well, as these are hard to get out of the pan sometimes!
In a large bowl, combine the melted butter with the sugar, using a whisk, and slowly add the beaten eggs. In a smaller separate bowl, mix the dry ingredients, flour through salt, and then slowly add those to the wet, whisking and then switching to a spatula. Don't over-mix - stir only until fully combined.
Cover the bottom of the lined baking dish with a thin layer of the brownie batter - you want to reserve more than half of the batter for topping. Lay the peppermint patties on the batter, slightly pressing to adhere, but not pushing down to the bottom of the baking dish. It's best to avoid the edges of the dish with the peppermint patties, as they can stick when baking if not surrounded by batter. Then add the rest of the batter making sure to completely cover all the candies. Any candies that are not covered with batter will get rock hard in the baking process.
Bake at 350 for about 38-45 minutes. It will be hard to tell with a knife if the batter is set, given there is melted candy inside. The best way to tell is to shake the pan gently and if the center is fairly set, you should remove the pan. You don't want to overbake these, and their thickness means carry over cooking will help solidify as they sit. Also, underdone brownies are just better. Cool COMPLETELY in the pan, on a baking rack, and then cut into small squares.