In a large bowl using a handheld or stand mixer fitted with the paddle attachment, beat the margarine, sugar, and brown sugar on medium/high speed until fluffy, about two to three minutes. Add the egg and vanilla, and beat on high speed until combined. Scrape down the sides and bottom of the bowl as needed.

In a separate bowl, whisk the flour, cocoa, baking soda, and salt together until combined. With the mixer running on low speed, slowly pour into the wet ingredients. Beat on low until combined. The cookie dough will be quite thick.

Switch to high speed and beat in the milk, then the chocolate chips. (The dough will be sticky.) Cover dough tightly and chill in the refrigerator for at least two to three hours. Don’t skip this step! Chilling is mandatory for this dough.

Remove cookie dough from the refrigerator and allow to sit at room temperature for 10 minutes.

Preheat oven to 350 degrees Fahrenheit. Line two baking sheets with parchment paper.

Form balls using a medium-sized scooper or a one-and-a-half-tablespoon measure. Arrange the cookies two to three inches apart on the baking sheets. Bake for 11 to 12 minutes or until the edges appear set and the centers still look soft.

Cool cookies for two minutes on the baking sheet. Press one mint thin on top of each warm cookie. Let sit for five minutes and then use a knife, spatula, or toothpick to gently swirl/spread the melted chocolate. Let the cookies cool and the chocolate harden completely.