Spread the chocolate ice cream into an even layer in the bottom of the springform pan. Place in the freezer for 30 minutes. Spread the hot fudge sauce over the chocolate ice cream (if it had been chilled, you'll need to rewarm briefly), then sprinkle the chopped Oreo cookies, pressing lightly so that they adhere to the hot fudge. Return to the freezer for at least 30 minutes. Lastly, dollop the vanilla ice cream over the chopped Oreos and gently spread into an even layer. Freeze for at least 8 hours, or overnight.
At least 1 hour (and no more than 12 hours) before serving, unmold the cake. To release the cake from the pan, invert the cake onto a plate or cardboard round the same size and wrap a hot kitchen towel around the cake pan, keeping it there for 30 seconds, or until the cake loosens, and slide the pan off. Remove the bottom of pan. Top with a serving plate and turn the cake right side up. Return to the freezer for at least 10 minutes.
While the cake is in the freezer, make the whipped cream. Combine the heavy cream, sugar and vanilla extract in the bowl of a stand mixer. Using the whisk attachment, whip on medium-high speed until the cream is light and fluffy and holds a medium to firm peak, 2 to 3 minutes.
Spread the whipped cream on the cake, working quickly so that the ice cream doesn't melt. (If it gets soft, just pop it back into the freezer for 5 minutes.) I used a Wilton #21 decorating tip to pipe a shell border around the top and bottom of the cake. Decorate with sprinkles on top, if desired.
Remove the cake from the freezer 5 minutes before serving to soften it slightly. A hot, dry knife will yield the cleanest cuts. Dip your knife in a glass of hot water, wipe it clean with a towel, and cut a slice. Repeat between every slice.
You can substitute 2 cups of store-bought hot fudge for the homemade.