Preheat oven to 325 degrees F and line baking sheets with silicone baking mat or parchment paper.
Whisk together flour, salt, baking powder and cocoa, and set aside. Beat the butter on medium-high until light and add sugars, creaming well. Add the vanilla and mint extract and beat until smooth. Add the flour mixture in 2 batches, alternately with the milk in one batch, mixing well. Stir in the mint chips. Chill dough for at least 15 minutes. You can make the dough a few hours or 2 days before baking and it will be fine. Just wrap it up and keep in the fridge.
Scoop the dough onto a parchment cookie sheet, press down on them slightly, and bake for 14-15 minutes…for a normal size cookie. Let the cookies cool completely on the cookie sheet, they will set up as they cool. Store in an air-tight container, they will stay fresh for a couple of days.