In a spring form pan (I used a 9 inch), layer the chocolate ice cream in the bottom. Smooth and return to the freezer to slightly harden about 1-2 hours.
Remove the cream filling from the Oreos and put the chocolate cookies into a bag. Smash the cookies in the bag until they are course crumbs. Pour cookie crumbs over chocolate ice cream and press in slightly. Pour the hot fudge (which may need to be slightly warmed so it is easier to work with) over the cookie crumbs and chocolate ice cream. Smooth and return to freezer for 1-2 hours until set.
Add vanilla ice cream over the top of the cake. Smooth and return to freezer 2-4 hours until completely set.
Make whipped cream for decorating by whipping together heavy cream, powdered sugar, and vanilla until stiff. Use some of whipped cream to cover sides and top of cake to hide chocolate and fudge layer. With the remaining cream, decorate using a piping bag and tip. Finish cake with colored sprinkles on top.
*Cover the bottom of the spring form pan with plastic wrap. It helps in transferring the cake to a plate or platter for decorating and cutting.
*Use a hair dryer on a low setting and slowly move around the outside of the spring form pan once or twice before attempting to remove the cake for decorating. The heat helps to release the ice cream from the sides.