Line a rimmed baking sheet with parchment paper or wax paper. Set a cold spring form (9-inch or 10-inch) pan ring (no bottom) on top.
In a standing mixer fit with the paddle attachment, beat vanilla ice cream until fluffy but still frozen, about 2 minutes. Spread thickly around the sides of the springform pan, then use the rest to cover bottom of pan.
Top evenly with crushed cookies, pressingly the crumbs lightly in to the vanilla ice cream.
Carefully heat the fudge sauce just enough for it to pourable. Pour evenly over the cookie crumbs. Freeze at least 8 hours or overnight.
In a standing mixer fit with the paddle attachment, beat chocolate ice cream until fluffy but still frozen, about 2 minutes. Spread on top of the fudge-cookie layer, smoothing the top (should be even with top of springform pan).
Cover with parchment paper or wax paper. Freeze at least 4 hours or overnight.
To unmold the cake:
Peel the top layer of parchment away. Place a large plate or serving platter on top of the cake. Gently flip the cake and plate over so the cake is right-side up on the plate (chocolate layer should be on the bottom, vanilla on the top). Peel the second piece of paper away.
Gently loosen the springform pan and wiggle it until it slides smoothly off the cake. Freeze while preparing the whipped cream.
To decorate the cake:
In a standing mixer with the whisk attachment, whip together cream, powdered sugar, and vanilla until soft peaks form. Frost the cake completely.
Use a piping bag with extra whipped cream to add flowers or other designs if desired. Or, top with sprinkles and maraschino cherries.
The cake can be eaten immediately, but for best results freeze at least 8 hours or overnight before serving. Cover and freeze up to 1 month in advance (or keep it in the freezer and continue eating it for up to 1 month).
One package OREOs has 36 cookies and yields about 3 cups crumbs.