Preheat oven to 350-degrees. Line an 8"x8"x2" inch baking dish with foil, extending foil over the pan edges; set aside.
In a medium bowl combine crushed cookies and melted butter. Press mixture into the bottom of prepared pan. Bake for 8 minutes. Cool slightly in pan on a wire rack.
Meanwhile, combine powdered sugar and softened butter in a food processor. Cover and process until butter is evenly distributed. With processor running, add milk and peppermint extract to make a paste.
Spread peppermint mixture evenly over the prepared crust. Freeze for 45 minutes.
In a microwave-safe bowl combine chopped chocolate and shortening. Microwave on high for 1 minute or until chocolate is melted, stirring once. Let chocolate cool for 15 minutes; pour over mint layer and spread evenly.
Using foil, lift the uncut bars out of the pan. Use a 1 1/2 inch round cookie cutter to cut mixture into rounds or cut mixture into squares. Makes 30 cookies.