Using an electric or stand mixer, beat together butter and both sugars until creamy. Add egg, egg yolk, and vanilla. Beat until combined.
Add in baking soda, baking powder, salt, cocoa powder and 1¼ cups of the flour, and beat slowly until combined. Add in last cup of flour and milk, and beat until thoroughly combined.
Add the baking chips and mix on low a few seconds until they are incorporated into the dough.
Place rounded tablespoons of dough on a baking sheet, about an inch apart.
Bake cookies for 10 minutes. Let cool a few minutes on the baking sheets, then move to a cooling rack.
Recipe by Flying on Jess Fuel at https://www.jessfuel.com/andes-mint-chocolate-chunk-cookies/