Mix the Ingredients: Add the milk, heavy whipping cream, sweetened condensed milk, vanilla extract, and salt to a large pitcher. Whisk until the condensed milk is fully blended and the mixture is smooth.
Fill the Ninja SLUSHi: Pour the mixture into the Ninja SLUSHi, keeping the amount between the 16-ounce minimum and 64-ounce maximum. Close the lid and select the MILKSHAKE preset. Do not add ice cubes or solid ice cream.
Freeze the Slush: Let the Ninja SLUSHi run for about 15–60 minutes, depending on the amount and starting temperature. When the machine beeps and the temperature lights stay solid, check that the slush is thick, cold, and creamy.
Dispense and Serve: Place a glass under the dispenser and slowly pull the handle to pour. Serve the ice cream slush right away. Let it run longer when the mixture is still too thin.