Preheat oven to 350 degrees Fahrenheit, line a 9?x13? cake pan with foil, allowing it to come over the edges, then coat foil with nonstick cooking spray.
For the brownies, in a large microwave-safe bowl, melt butter and chocolate chips together in 30 second increments, stirring between. Stir mixture until smooth. Quickly whisk in (by hand) brown sugar, white sugar, vanilla, eggs, and milk, just until incorporated (don’t over-mix or brownies will be tough and lose their nice texture). In a separate bowl, stir together flour, baking cocoa, baking powder, salt and corn starch. Add to chocolate mixture, along with 1 sleeve crushed Thin Mints, and stir just until incorporated (again, don’t over-mix!). Pour and spread mixture into prepared pan and bake for approximately 15-16 minutes, or just until a toothpick comes out mostly clean, with a few moist crumbs on it. You don’t want the toothpick to come out super gooey or completely clean, but moist crumbs are what you’re shooting for. Don’t over bake or they’ll lose their fudgy texture.
Let brownies cool in pan on a cooling rack for 20 minutes, then place in the fridge to cool completely.
To make the mint frosting, in the bowl of an electric mixer, cream together powdered sugar, melted butter, milk, peppermint extract, and green food coloring until smooth and creamy. Spread over cooled brownies and sprinkle with Thin Mint Cookies. Place in the refrigerator until ready to serve. Carefully remove foil from pan, release foil from brownie edges, then slice into squares and serve.
