In a 6-quart container with lid, refrigerate milk overnight. The following day, bring the mixture to room temperature and leave for 8 to 10 hours to develop acidity and culture. Using a ladle, skim cultured cream into churning vessel and reserve skim milk for other uses. Churn for 45 to 60 minutes, or until butter fat begins to separate from buttermilk.
In a chinois lined with cheesecloth, strain mixture and squeeze to remove excess moisture. "Rinse" butter in a container of ice water. Transfer rinsed butter to a perforated pan set over a deep hotel pan to drain and remove any ice chips.
Using a butter board, press out excess moisture using a butter paddle. If using a perforated pan, massage and knead butter by hand until moisture is removed. Wrap tightly and store refrigerated, or freeze if holding more than 5 days.