First, line a rimmed baking sheet with parchment paper or wax paper. Set a cold springform (9-inch or 10-inch) bottomless pan ring on top.
In a standing mixer, using the paddle attachment, beat the vanilla ice cream until it's fluffy but still frozen, about 2 minutes. Spread thickly around the sides of the springform pan, and then spread the rest inside the pan, making up the bottom layer.
Top evenly with crushed cookies, pressingly the crumbs lightly into the vanilla ice cream.
Carefully heat the fudge sauce just enough for it to pourable- a few seconds in the microwave. Pour evenly over the cookie crumbs. Freeze at least 8 hours or overnight.
In a standing mixer, using the paddle attachment, beat the chocolate ice cream until it's fluffy but still frozen, about 2 minutes. Spread on top of the fudge-cookie layer, smoothing the top. It should now be even with the top of the springform pan.
Cover with parchment paper or wax paper. Freeze at least 4 more hours.