Preheat your oven to 350°F.

Line a 13x9-inch baking pan with foil, extending the foil over the edges.

For the crust, place the chocolate sandwich cookies in a food processor.

Cover and process until very finely chopped.

In a medium bowl, combine the chopped cookies and melted butter.

Press this mixture into the prepared pan.

Bake for 12 minutes or until the crust is set.

Cool it in the pan on a wire rack.

While the crust is cooling, prepare the peppermint filling.

In a large bowl, beat the powdered sugar and softened butter with a mixer on low until the butter is evenly distributed.

Add the milk and peppermint extract and beat until smooth.

Spread the peppermint filling over the cooled crust and place it in the freezer for 45 minutes.

In a medium bowl, combine the chopped semisweet chocolate and shortening.

Microwave for 1 to 2 minutes or until the chocolate is melted and smooth, stirring every 30 seconds.

Allow the melted chocolate mixture to cool for 15 minutes.

Spread it over the peppermint filling.

Chill for another 15 minutes or until the chocolate is firm.

Using the edges of the foil, lift the uncut bars out of the pan and cut them into your desired serving sizes.