Combine cream and buttermilk together in a glass bowl. Stir to combine and cover loosely with a kitchen towel.

Let the bowl sit out at room temperature for 24-48 hours. When the cream is thick and tangy, move the bowl to the refrigerator.

Allow the cream mixture chill for 2-3 hours before churning.

Place the chilled cream mixture in the bowl of a stand mixer with a whisk attachment.

Whip on medium-high until the mixture forms stiff peaks and looks like whipped cream.

Lower the speed to medium-low and cover the stand mixer with a kitchen towel to minimize splattering. Whip until the butter comes together and separates from the buttermilk.

Pour the butter and buttermilk into a fine-mesh sieve lined with cheesecloth to strain the butter. Wrap the cheesecloth around the separated butter and wring the cheesecloth to remove as much of the buttermilk as possible.

Save the buttermilk! Use it for baking, marinades or to culture the next batch of butter.

Transfer the butter to a bowl filled with ice water and wash the butter repeatedly by squeezing the butter ball to release excess buttermilk. Wash the butter till the water is clear, about 3-4 washes.

At this point mix in salt if using. This will prolong its shelf life or skip this step and shape into a butter block or however you like. Wrap in wax or parchment paper and store in the fridge for a month or freeze for later. Enjoy!