Line an 8-inch cake pan with plastic wrap and scoop ice cream into the pan. Spread into an even layer, cover with plastic wrap and transfer to freezer to harden, about 2 hours.
Prepare the 8-inch cake layer if you haven’t already. Freeze any leftover layers or use in another recipe. Cool cake completely.
When the cake is cooled and the ice cream layer is hardened, start to assemble. Place chocolate cake on a serving plate. Spread chocolate fudge sauce on top leaving about 1/2-inch of space around the edges. Then sprinkle on the Oreo crumbs. Uncover the ice cream layer and place it on top of the cake with toppings. Cover the entire cake loosely with plastic wrap and place the cake back in the freezer while preparing the frosting.
For the whipped cream frosting, add cold heavy cream, confectioners’ sugar and vanilla to a cold metal bowl of a stand mixer fitted with a whisk attachment (or use a hand mixer). Beat on low speed to combine the ingredients, then increase speed to medium-high and mix until soft peaks form, about 1 ½ to 2 minutes (don’t overmix).
Take the cake out of the freezer and cover it with whipped cream. Transfer remaining whipped cream to a frosting bag and pipe whipped cream around the edge of the cake. Garnish with crushed cookie crumbs. Slice and serve or store in the freezer until ready to serve. Let the cake sit on the counter for 10 to 15 minutes so it’s easier to slice.