In a 9-inch springform or silicone pan, layer the chocolate ice cream in the bottom. Smooth and return to the freezer until set, about 1 hour.
Pour Oreo crumbs over chocolate ice cream and press in slightly. Pour the hot fudge over the cookie crumbs. Smooth and return it to the freezer until set.
Add vanilla ice cream over the top of the cake. Smooth and return it to the freezer until completely set.
Remove the ice cream cake from the freezer and let it sit at room temperature for 5 minutes before removing the cake for decorating.
Use whipped topping to cover sides and top of cake. Drizzle chocolate ganache on top (if using) and decorate with additional cookie crumbs if desired.