In a large mixing bowl, combine sweetened condensed milk and dry brownie mix. Beat until well incorporated. Add thawed container of Cool Whip. Filling must be well mixed; use a spatula to overturn the mixture from the bottom to the top if necessary.
Break brownies into chunks and mix into ice cream. Save a handful to sprinkle on top.
Microwave hot fudge sauce for 30 seconds. Spoon some onto the top of the ice cream and swirl with a knife.