Combine cream and buttermilk in a clean, closeable container.
Close the container, place it in a cool room-temperature spot, and leave it for 24 hours.
Check if it’s thick and smells wonderfully buttery.
Put it in the fridge overnight to allow the fat to gradually solidify.
Take the cream out of the fridge and let it warm up to 55°F (12°C).
Transfer the cream to a food processor or stand mixer and process until it turns into lumps of butter.
Line a fine-mesh strainer with cheesecloth and pour the butter mixture through it into a bowl.
Lift the cheesecloth and twist to press out more buttermilk.
Transfer the cheesecloth-wrapped butter to an ice bath until firm.
Remove butter from the cheesecloth and stir to force out additional buttermilk.
If desired, knead in ¼ teaspoon salt and transfer to an airtight container.
