In a medium bowl, whisk together the milk, cocoa powder, sugar, vanilla extract, and salt until the sugar dissolves completely.
Gently fold the thawed whipped topping into the milk mixture using a spatula. Mix until just combined; do not overmix.
Pour the mixture into a shallow freezer-safe dish, like a loaf pan.
Freeze for at least 3 to 4 hours, stirring vigorously every hour for the first 2 hours to break up ice crystals and keep it smooth.
After the final hour of stirring, let it freeze undisturbed for another 1 to 2 hours until it reaches a thick, soft-serve consistency.
Serve immediately with a spoon or a wide straw.
Notes
For a richer chocolate flavor, you can substitute 1/4 cup of the milk with 1/4 cup of melted semi-sweet chocolate chips, cooled slightly before mixing.
If your Frosty becomes too hard, let it sit on the counter for 10 to 15 minutes before serving.
You can slightly adjust the sugar to match your preferred sweetness level.