Mix and bake brownies as directed on box in a 9x13 pan.
Soften both ice creams.
Scoop chocolate out onto brownie, then spread it out in an even layer.
Warm hot fudge and pour over chocolate ice cream. Crush Oreos and spread over fudge.
If desired at this point you can freeze cake for 1 hour to make adding the next layers easier.
Scoop peanut butter ice cream over the Oreos and spread in an even layer.
Top with whipped topping and sprinkles, if desired.
Cover and freeze for at least 8 hours (or up to 24 hours) until firm.
Slice and serve. If cake has been frozen for 24 hours or more, you may need to let it rest on the counter for 15 minutes to soften a bit before slicing.