Combine cream and buttermilk in a clean, closeable container (the carton from the cream works well).

Close the container, place it in a cool room-temperature spot, and leave it for 24 hours.

Check it and see if it’s thick and smells wonderfully buttery.

When it’s gotten as tangy as you like it, put it in the fridge overnight (or longer).

Take the cream out of the fridge and let it slowly warm up to 55°F (12°C).

Transfer the cream to a food processor, or a stand mixer with the whisk attachment, and process.

Over the course of 3 minutes or less in the food processor, or longer in stand mixer, the whipped cream will become grainy with bits of butter and then turn into lumps of butter splashing in liquid.

Stop the machine immediately.

Line a fine-mesh strainer with a triple layer of cheesecloth, leaving a few inches of cloth hanging over the sides.

Pour the butter mixture through your cheesecloth-lined strainer 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 over strainer to press out more buttermilk.

When butter starts to squeeze through the cloth, you are done.

Transfer the cheesecloth-wrapped butter to your ice bath and let it rest there till it’s firm to the touch, about 2 minutes.

Pour the buttermilk into an airtight container to save it.

Remove butter from the cheesecloth and transfer it to the now-empty bowl.

Stir and press with a wooden spoon for 1 to 2 minutes to force out additional buttermilk.

Drain this buttermilk from the bowl, add to the buttermilk container, and refrigerate until ready to use.

If you want salted butter, knead ¼ teaspoon salt into butter with the wooden spoon.

Transfer butter to separate airtight container and refrigerate until ready to use.