Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper.
Cream the butter and sugars together in a large bowl for 2–3 minutes, until smooth and fluffy.
Add egg, yolk, vanilla, and peppermint extract and mix until fully combined.
Whisk dry ingredients (flour, cocoa, baking soda, salt) in a separate bowl.
Combine dry ingredients with the wet mixture and mix until a soft dough forms.
Fold in chopped Andes mints and chocolate chips.
Scoop dough into 2-tablespoon portions, roll into balls, and place on baking sheets 2 inches apart.
Chill for 20–30 minutes before baking (optional but helps keep them thick and chewy).
Bake for 9–11 minutes, until edges are set but centers look slightly underbaked.
Press a few extra Andes mints on top right after baking for a melty, glossy finish.
Cool on the pan for 5 minutes, then transfer to a rack to finish cooling.