How to Make It
Preheat oven to 375°F with oven racks in the top third and bottom third of oven. Combine chocolate chips and butter in a large microwave-safe bowl, and microwave on MEDIUM (50% power) until melted and smooth, about 2 minutes, stirring every 30 seconds. Whisk in sugar, eggs, vanilla, and 1/4 teaspoon of the mint extract until smooth. Whisk together flour, cocoa, baking powder, and salt in a medium bowl. Stir flour mixture into chocolate mixture; fold in chopped mints. Drop dough by tablespoonfuls 2 inches apart onto 2 parchment paper-lined baking sheets.
Bake in preheated oven until cookies are just set on top, about 10 minutes, switching pans top rack to bottom rack halfway through baking. Remove pans to wire racks, and immediately flatten domed cookie tops with the bottom of a heat-proof dry measuring cup. Cool cookies on pans 5 minutes; transfer cookies to wire racks, and cool completely, about 30 minutes.
Whisk together powdered sugar, 3 tablespoons of the milk, and remaining 1/4 teaspoon mint extract in a small bowl. Stir in up to 1 more tablespoon of milk, 1 teaspoon at a time, until desired consistency is reached. Transfer 2 tablespoons of the icing to a separate small bowl, and stir in desired amount of green food coloring.
Spoon remaining white icing into a ziplock plastic freezer bag with 1 corner snipped or a piping bag. Pipe circles of white icing over cooled cookies. Add a little of the green icing onto white icing circles, and swirl with a wooden pick to create freeform circles. Let stand at room temperature until set, about 1 hour.