Place cold chocolate milk, a can of sweetened condensed milk, and Cool Whip into a blender and blend until smooth.
Cover blender with plastic wrap and freeze the chocolate mixture for 2 hours, up to 3. The blender can be added to freeze if your blender is freezer safe like ours. Otherwise, pour into a freezer-safe container.
Once the mixture is half frozen in the blender, remove the container from the freezer.
Scrape down the sides of the frosty mixture (and any ice crystals that have formed) and blend again.
Pour Copycat Wendy’s Frosty into serving glasses and serve.
Do not substitute evaporated milk for sweetened condensed milk – it’s not the same thing and is not interchangeable.
You can double these ingredients; however, most blenders won’t hold a whole gallon of milk plus additional ingredients.
This frozen beverage needs time to freeze, so while it only takes a few minutes to prep, it still needs between 2-3 hours to freeze for the correct Frosty consistency.