Mix the chocolate base: Pour the chocolate milk into a large bowl. Add the sweetened condensed milk and whisk until well combined. Add the whipped topping and whisk until smooth.
Add to the Ninja SLUSHi: Transfer the mixture to a measuring cup or pitcher. Pour it through the fill port into the Ninja SLUSHi vessel, keeping the mixture between the MIN and MAX fill lines.
Choose the setting: Close the cover, turn on the machine, and select the Milkshake preset. Turn the cold level all the way up for a thicker texture. Let the machine run for about 60 minutes, or until the drink reaches your preferred consistency.
Dispense and serve: When the LEDs turn solid and the machine beeps, place a cup under the dispenser and slowly pull the handle. The Frosty can become very thick, so serve it with a spoon if needed.