Combine all ingredients in a large bowl or plastic container with a lid. Place the covered container into the refrigerator and chill for 3 hours.
Pour the mixture into a crank freezer or electric ice cream maker and follow the manufacturer’s directions. If you do not have an ice cream maker, you can improvise by filling a quart-size zipper-style bag two-thirds full with the buttermilk mixture and zipping it closed, then putting it into a gallon-size zipper-style bag. Hold the mouths of the bags flush while somebody else fills the gallon-sized bag with crushed ice and rock salt, in layers. Zip the gallon-sized bag closed and massage and shake the bags to achieve the same effect as from a classic ice cream freezer. You may have to do a few batches this way. Keep checking the quart bag to see if your ice cream has frozen sufficiently.
Pack ice cream into a quart-size lidded container. Cover with plastic wrap, pressing it onto the surface of the ice cream. Place the ice cream into the freezer and leave until fully set, about 5 to 6 hours, or overnight.