Combine milk, half-and-half, sugar, and vanilla extract in a pint-size resealable plastic bag; seal tightly.
Put a scoop of ice, 3 tablespoons ice cream rock salt, and the bag containing the milk-cream mixture into a gallon-size resealable plastic bag; seal tightly.
Rock the bag back and forth (do not shake) until contents thicken into ice cream, about 10 minutes. Wipe salt from the top of the pint-size bag before opening to prevent salt from getting into the ice cream.
Nutrition data for this recipe includes the full amount of salt used for freezing. The actual amount of salt consumed is negligible.
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