In a food processor, pulse flour, sugar, cocoa powder, salt, and baking soda until combined. Add margarine, milk, vanilla, and mint extract. Pulse a few times until mixture comes together. Transfer mixture to a large bowl and knead with your hands in the bowl for one minute. Chill the dough in the refrigerator for 1 hour.
2Preheat oven to 350 degrees F and line two large baking sheets with parchment paper or silpats. Remove dough from refrigerator. Roll a heaping teaspoon of cookie dough into a ball and place onto prepared baking sheets, leaving about 3 inches in between each ball. Evenly flatten the dough with your fingertips so that it is about ¼ inch thick and bake for 12-14 minutes. Let cool completely.
Melt chocolate chips and margarine over a double boiler or in the microwave. Stir in the mint extract and mix until smooth. Dip completely cooled cookies into the chocolate and remove with a fork, gently scrapping off excess chocolate using the side of the bowl. Or, spread a thin layer of the chocolate on top of the cookies. Place cookies on a parchment-lined plate or tray and refrigerate until chocolate coating sets. Store in the refrigerator.