Preheat oven to 350 degrees. Butter a cookie sheet and set aside. In a large mixing bowl, whisk together brownie mix, flour, brown sugar and salt. Add in eggs and milk and stir with a large spoon. Add in melted butter, vanilla and peppermint extract, stir until well blended. Spoon dough by heaping tablespoonfuls onto greased cookie sheet (you can fit 12 per sheet). Bake for 10 minutes. Remove from oven and allow to cool completely before frosting. Once cool, frost each cookie and sprinkle with crushed candy canes.