Place a layer of parchment paper over the bottom of a 9-inch springform pan, and lightly grease the sides. (You can cut a round piece to fit the bottom of the pan or, better yet, anchor a larger sheet with the ring of the pan, allowing the parchment to stick out the sides as shown in the prep photo. See Tip #2 and #3, below.)
Spread the vanilla ice cream in an even layer over the bottom of the pan, making sure to fill all the edges. Place in the freezer for at least 30 minutes. Sprinkle the cookies evenly over the vanilla ice cream, lightly pressing them into the ice cream. Spread the hot fudge sauce over the cookie layer (if it’s chilled, you’ll likely need to warm briefly—but avoid making it too hot). Return to the freezer for at least 30 minutes more. Next, evenly spread the chocolate ice cream over the fudge layer. Freeze for at least 8 hours or overnight. Tip:Place your serving platter in the freezer now, too. It helps to have a very cold surface for the next step.
At least 1-2 hours before serving, unmold the cake. To easily release the cake from the pan, invert it on a platter and wrap a hot kitchen towel (I run very hot water over the towel and then quickly wring it out) around the sides of the pan, keeping it there just long enough to loosen the cake from the ring. Remove the ring, and then pry off the bottom of the pan with the help of the parchment paper. Immediately return the cake to the freezer for at least 30 minutes.
Spread the whipped topping on the cake, working quickly so that the ice cream doesn’t melt. (If it gets soft—which can happen quickly, especially when the weather is warm—return it to the freezer to firm up.) Decorate with reserved cookies or sprinkles, if desired, and return to the freezer.
Remove the cake from the freezer 5-10 minutes before serving (depending on how long it’s been frozen and how warm/cool the room is), to soften it enough to slice. A hot, dry knife will produce the cleanest cuts. To do this easily, dip the knife in a tall glass of hot water, wipe it clean with a towel, and cut a slice. Repeat before cutting the next slice. Enjoy!