Preheat your oven to 350°F (177°C) and line a baking sheet with parchment paper.
In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy, about 2-3 minutes.
Beat in the egg, vanilla extract, and peppermint extract until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Fold in the crushed peppermint candies and chocolate chips.
Scoop dough by the tablespoon onto the prepared baking sheet, spacing cookies about 2 inches apart.
Bake for 8-10 minutes, or until edges are set and centers are slightly soft.
Let cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.