Mix buttermilk and cream together in a container with a lid. Let it sit for 24 hours at room temperature.

Place the cultured mixture in the refrigerator for 24 hours.

Pour the mixture into a stand mixer with a whisk attachment. Let the machine run until the mixture separates into butter and buttermilk, this takes about 20 minutes.

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Press the liquid out of the butter and wash it in cold water.

Place the unsalted butter in a bowl and mix in the desired amount of salt.

The butter is ready to use and will keep for a few days in the fridge.