Prepare the flavor base: In a small bowl, combine the heavy whipping cream, sweetened condensed milk, vanilla extract, and salt. Whisk or gently stir until just combined. Do not overmix at this stage.

Portion into the jar: Pour the ice cream mixture evenly into two small (8-ounce or half-pint) mason jars, filling them about halfway. If adding mix-ins like chocolate chips, stir them in now.

Prepare the freezing environment: Fill a large (quart-sized) mason jar about halfway with ice cubes. Pour the rock salt over the ice. The salt is crucial for making the ice cold enough to freeze the cream.

Assemble the freezing unit: Place the lids tightly on the two smaller jars containing the ice cream base. Place these smaller jars inside the large jar filled with salted ice. The lids of the small jars should be above the level of the outer ice/salt mixture.

Shake vigorously: Secure the lid on the large jar. Wrap the entire jar in a kitchen towel or wear oven mitts (it will get very cold). Shake the jar vigorously and consistently for 5 to 7 minutes until the mixture inside the small jars noticeably firms up into soft-serve consistency.

Serve or harden: Carefully remove the small jars from the ice bath. Wipe down the outside of the small jars before opening them. Serve immediately for soft-serve, or seal the lids tightly and place the jars in the freezer for 1-2 hours for a firmer scoopable texture.