Sift the confectioners' sugar through a fine-mesh sieve into a large bowl. Add the heavy cream, mint extract and green food coloring if using. Beat with an electric mixer until stiff peaks form, about 4 minutes.
Lay out 6 cookies on a large flat plate or baking sheet, evenly spaced apart. Put 1 heaping tablespoon of the mint cream onto each cookie. Repeat to make 3 more layers of cookie and mint cream, lightly pressing down between each layer so that the cream just comes to the edge of the cookies. Top each cake with 1 last cookie.
Cover loosely with plastic wrap and refrigerate for 12 hours or overnight.
When ready to serve, carefully transfer the cakes to a rack set over a rimmed baking sheet. Combine the chocolate, shortening and a pinch of salt in a small saucepan over medium-low heat, stirring constantly with a rubber spatula, until melted, about 4 minutes. Transfer to a small bowl and allow to cool just slightly. It is ready to use when the chocolate is no longer hot but is still melted.
Pour about 2 tablespoons of the chocolate on top of each icebox cake (use any of the excess chocolate coating that drips onto the baking sheet). Freeze the icebox cakes to set the chocolate, about 10 minutes. Transfer each cake to a plate and serve.
Copyright 2014 Cooking Channel, LLC. All rights reserved.