How to Make It
Prepare the Cookies: Preheat oven to 350°F. Sift flour, cocoa, baking soda, and salt into a medium bowl, set aside.
Beat together butter, brown sugar, and granulated sugar in the bowl of an electric stand mixer fitted with a paddle attachment on medium speed until fluffy, about 3 minutes. Add egg, and blend to combine. Turn mixer to “stir”; slowly add flour mixture, and beat until combined. Turn mixer to medium speed; slowly add buttermilk, and beat just until combined. Stir in vanilla and mint extract.
Using a 1 ounce ice cream scoop, scoop 30 rounded scoops, 2-inches apart, on baking sheets lined with parchment paper. Bake in preheated oven until slightly cracked and cookies slightly spring back, 10 to 12 minutes. Let cool on baking sheets set on wire racks.
Prepare the Filling: Beat together cream cheese and butter with an electric mixer on medium speed until slightly fluffy. Gradually add powdered sugar until well incorporated. Add vanilla, mint, salt, and 3 drops of green food coloring to achieve a pale green color. Stir in crushed Thin Mints cookies until incorporated. Place filling in large zip top bag, and snip the corner. Pipe filling evenly on 15 of the cookies. Top with remaining cookies.