Preheat your oven to 350 degrees.
Using a stand mixer fitted with the paddle attachment or a hand mixer beat softened butter and brown sugar on high for 1-2 minutes. Scrape down the bowl once complete.
Add in milk, egg, and melted chocolate. Mix until combined. Scrape down the bowl once complete.
In separate bowl, whisk dry ingredients together (flour, cocoa powder, baking soda, salt).
Add dry ingredients to wet ingredients in 3-4 portions. Mix until combined.
Using a wooden spoon or spatula, mix in chopped Andes Mints.
Grease and line baking sheets with parchment paper or silicone baking mat.
Roll dough into small balls, ¾-1 inch in diameter. If the dough is too sticky to roll, place it in the fridge for 30-60 minutes to allow it to set.
Place the dough balls on to baking sheet, leaving at least 1 inch in between each cookie.
Bake for 7-10 minutes, or until the cookies just begin to crack.
Remove the cookies from the oven and let them cool to room temperature to just slightly warm, then place one Andes Mint on top of each cookie and press down.