Line bottom of pan with parchment paper for easy release.
Spread thin layer of mousse on the bottom of pan. Cover with half the cookies in a single layer.
Spread half the mousse over the cookies. Repeat layers with remaining cookies and mousse.
Freeze until firm then push up the pan bottom and carefully place mousse cake onto serving plate. Garnish with additional whipped topping and cookie crumbs, if desired.
Notes
Use a cake collar to prevent the sides of the cake from sticking to the sides of a springform pan. Cake collars aren't needed for push-up pans.Add garnish. Top with piped whipped cream. Then process one or two extra cookies in a food processor to a fine crumb to sprinkle over the top. Or use black cocoa powder for garnishing.