In a large bowl, combine melted butter, powdered sugar, milk, peppermint extract, and a drop of green food coloring if desired.
Stir in graham cracker crumbs until well mixed.
Press mixture firmly into a greased or parchment-lined 8x8-inch baking dish.
In a microwave-safe bowl, combine chocolate chips and butter. Microwave in 30-second intervals, stirring in between, until melted and smooth.
Pour melted chocolate over the mint layer and spread evenly.
Refrigerate for at least 2 hours or until set.
Cut into bars and enjoy!