Prepare the cake mix into two 8-inch cake rounds, bake, and let cool completely. Wrap each cake round with plastic wrap and place in the freezer. (Only one of the cake rounds will be used in this recipe. See note.)
Line the same 8-inch cake pan with plastic wrap and spread the softened ice cream evenly into the pan. Top with plastic wrap and freeze until set, about an hour.
Coarsely crush or chop 20 Oreo cookies and combine in a medium bowl with 1 cup of room temperature hot fudge sauce.
Cut one of the cake rounds in half the long way to form two thinner cake layers.
Place one layer of the chocolate cake on your cake stand or plate. Then spread half of the Oreo fudge mixture on top. Then carefully place the ice cream layer on top and cover it with the remaining half of the Oreo fudge mixture. Top it all off with the final cake layer.
Frost the cake with the whipped topping. Decorate with Oreo cookies and more hot fudge sauce or chocolate sauce, if desired.
Place cake back in the freezer until set, usually takes a few hours. Remove cake from freezer about 10-15 minutes before slicing and serving.
The cake mix makes two chocolate cake rounds, but only one will be used in this recipe. Keep the other wrapped tightly in plastic wrap and store it in the freezer for up to 5 months.