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.
