1. Place heavy cream and buttermilk in a sterile jar; cover and shake just to combine. Let stand in a 65-to 72-degree room for 3 days.

2. Transfer cream mixture to the bowl of an electric mixer fitted with the paddle attachment and covered with a shield. Mix for 2 to 5 minutes until a ball forms. Drain buttermilk from bowl and keep refrigerated in a covered container.

3. Pour cold water over butter in bowl to rinse. Transfer butter to a clean work surface; using butter paddles, fold butter until softened and all of the liquid "weeps" from the butter, 20 to 30 minutes. Butter may be stored in a covered container, refrigerated, for up to 1 week.