Chocolate Sauce: Combine milk, chocolate, sugar, and butter in a sauce pan over medium heat and bring to a boil. Boil 15 minutes, stirring constantly. Cool and then chill in the refrigerator.
Put the cookies (including the creamy middles) in the food processor and process for about 30 seconds, until you have medium crumbs. Pour the crumbs into the 9x13 pan and stir in the melted butter. Press the mixture into the bottom of the pan and freeze for an hour or two, until pretty solid.
On top of frozen cookie crust spread all of the softened ice cream. Press plastic wrap on top of the ice cream and freeze for about three hours, until ice cream is solid.
Pour cooled chocolate mixture over the ice cream. Freeze for about three hours, until the layer is solid. This is important, otherwise the sauce will run everywhere when you cut and serve. Spread whipped topping over the top and sprinkle with nuts. Keep frozen.
Preparation - Two Day Method
You can make the cookie crust and the ice cream layer on one day, then freeze overnight and make the chocolate sauce the next day. Just make sure you allow enough time for the chocolate sauce layer to freeze solid.