Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.,In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.,In a large bowl, beat the butter and sugar together until light and fluffy, about 3 minutes.,Add the egg, vanilla extract, and peppermint extract, beating until fully incorporated.,Gradually add the dry ingredients to the wet ingredients, mixing until just combined.,Fold in the mini chocolate chips and half of the crushed peppermint candies.,Scoop tablespoon-sized portions of dough and roll into balls. Place them on the prepared baking sheet, spacing them about 2 inches apart.,Press a few extra mini chocolate chips and crushed peppermint on top of each cookie for added decoration.,Bake for 8–10 minutes, or until the edges are set but the centers are still soft. Let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.