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 a stand mixer with the whisk attachment and process until it transforms into lumps of butter.
Line a fine-mesh strainer with cheesecloth and pour the butter mixture through it into a large bowl.
Fill a medium mixing bowl halfway with ice and water.
Lift the cheesecloth by its edges and twist and squeeze tightly to press out more buttermilk.
Transfer the cheesecloth-wrapped butter to the ice bath and let it rest for about 2 minutes.
Remove butter from the cheesecloth and transfer it to a bowl.
Stir and press with a wooden spoon for 1 to 2 minutes to force out additional buttermilk.
If desired, knead ¼ teaspoon salt into the butter.
Transfer butter to an airtight container and refrigerate.
