With a mixer, cream butter until light and fluffy, about 1 minute. Add in brown sugar and white sugar and mix until well combined.
Add in the egg and vanilla extract. Mix.
In a separate bowl, sift together flour, cocoa powder, baking soda, cornstarch, salt, and espresso powder. Once combined, slowly incorporate dry ingredients into wet ingredients. Mix until just combined.
Add in Andes mint baking bits and semi-sweet chocolate chips. Mix. Once combined, chill dough in refrigerator for about 20 minutes (this makes the dough easier to work with).
Preheat the oven to 350 degrees. Line a baking sheet with parchment paper or spray with cooking oil.
With a medium cookie scoop (about 2 Tbsp of dough per cookie), scoop the dough and spread evenly on the cookie sheets, about 2 inches apart.
Bake for 13-15 minutes, or until the cookies are done.
Enjoy!