In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt together. Set aside.
Using a handheld or stand mixer with a paddle attachment, cream the butter and sugars on medium speed for 2-3 full minutes, scraping down the sides of the bowl as needed. Beat in the egg and vanilla.
Slowly add to the dry ingredients to the wet ingredients. Fold in the chocolate chips and andes mint pieces. Cover and refrigerate for at least one hour.
Once chilled, preheat oven to 350°F. Line a large baking sheet with parchment paper or a silpat baking mat and set aside.
Scoop cookies using a large 3 -4 inch cookie scoop and place at least 2 inches apart on a parchment lined baking sheet.
Bake for 9-11 minutes. Cookies will appear undone and very soft. Allow to cool on the cookie sheet for at least 5 minutes before transferring to a wire rack to cool completely.