Preheat oven to 350F. Line a baking sheet with parchment paper and set aside.
Melt your stick of butter in the microwave until PARTIALLY melted (about 5-15 seconds if the stick is fresh from the fridge). It will be part liquid and the other part will be whole but soft. We don’t want it completely melted because that will make the cookies not as soft!
Combine the partially melted butter with the sugars and cream using a stand mixer or a hand mixer until well combined (about 2-3 minutes). Blend in the vanilla until incorporated.
Add in the egg and DO NOT OVERMIX! Mix until partially incorporated but you can still see the yellow of the yoke. This is also an important step because if over-mixing the egg, will create stiff cookies!
In a separate bowl, combine the flour, cocoa, baking soda, and salt. Whisk together until evenly distributed. Slowly add the dry ingredients into the wet ingredients while mixing. Add until all ingredients are incorporated and the batter is smooth. Add in the milk while mixing together.
Fold in the Andes Mints and chocolate chips. Add more or less chips depending on your preference.
Use a cookie scoop, or your hands, to roll together about 2 tablespoons of dough into a ball and place on the baking sheet fitted with parchment paper. You can sprinkle the balls with more Andes Mint chips if you desire.
Bake 10-12 minutes. Quickly transfer to a wire rack to cool.