1Position a rack in the middle of the oven and preheat to 350°F. Grease a 9-by-13-inch baking dish and line it with foil. Grease the foil.
2In a saucepan, warm the butter and chocolate over low heat, stirring often, just until melted. Remove from the heat and whisk in the sugars. Whisk in the eggs slowly, beating well. Whisk in the vanilla and salt.
3Sift the flour and cocoa over the chocolate mixture and, using a rubber spatula, stir in until just blended.
4Pour about 1/2 of the brownie batter into the prepared dish, spreading it into an even layer. Place the peppermint patties over the top in a single, even layer. Top with the remaining brownie batter and spread into an even layer.
5Bake until a toothpick inserted into the center comes out almost completely clean, about 30 minutes. During the last 3 minutes of baking, sprinkle the chocolate chips over the top in an even layer; return to the oven to let the chips melt enough to stick to the surface.
6Let cool completely in the dish, then remove from the dish using the foil to lift them out. Cool in the refrigerator until firm. Cut into gooey squares.