Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl. Set aside.
Using a stand mixer, cream together butter and sugars. Add peppermint extract and egg, then mix well.
Add flour to butter mixture in two batches, mixing just until combined. Chill dough in refrigerator for 1 hour.
Once dough has chilled, preheat oven to 350 degrees. Lightly spray two baking sheets with nonstick spray.
Form 1 tablespoon of dough into a ball (using hands), then roll in powdered sugar until completely coated. Repeat with remaining dough. Place balls on baking sheet at least 1-inch apart.
Bake cookies until they spread and the tops crinkle, about 10 minutes.
Cool cookies on baking sheet for 5 minutes, then transfer to a baking rack to cool completely.
Store cookies in an airtight container until ready to eat. Enjoy!
Notes
Feel free to use light or dark brown sugar for this recipe.For best results, bake one pan of cookies at a time. Doing so will ensure the cookies bake evenly.Be careful not to over-bake the cookies, or they will be dried out. The dark part of the cookie inside the crinkles may still look slightly underdone even after the exterior has set and crackled. However, the cookies will continue cooking a bit as they cool.