Set Up Your Ninja Slushi: Place the machine on a flat surface and allow it to sit upright for two hours before first use. Install the condensation catch, auger, vessel, bail handle, and drip tray according to the manufacturer's instructions.
Mix the Frosty Base: Add the Cool Whip, cold chocolate milk, and sweetened condensed milk to a large bowl. Whisk until the mixture is smooth and no white streaks or lumps remain.
Pour and Freeze: Pour the mixture through the easy-fill port, keeping the liquid below the machine's maximum fill line. Close the cover, turn on the machine, and select the Milkshake preset. Let the mixture freeze for 15–60 minutes, following the machine's indicators.
Dispense and Serve: Place a cup under the dispensing handle and pull it slowly to serve. Release the handle to stop the flow. Serve the Frosty immediately while it is thick and creamy.