1Heat oven to 350°F (325°F for dark or nonstick pan). Line pan with aluminum foil; grease bottom of pan with shortening. Make and bake brownies as directed on box, using butter and eggs, for (8- or 9-inch) square pan. Cool 1 hour.
2In medium bowl, stir together sweetened condensed milk and peppermint extract. Beat in enough powdered sugar on low speed of electric mixer until blended and slightly crumbly. Turn mixture onto surface sprinkled with powdered sugar. Knead lightly to form a smooth ball. Pat mixture evenly over top of brownies.
3Spread frosting over brownies. To serve, remove brownies from pan by lifting on aluminum foil; transfer to cutting board. With long sharp knife, cut into squares, 4 rows by 4 rows.