Pour 500ml of full-fat heavy cream into a stand mixer bowl or a large jar. The cream must be at room temperature (around 20°C) — cold cream takes much longer to churn.
Beat on high speed. The cream will go through stages: soft peaks, stiff peaks, and then suddenly it will break and turn grainy yellow — the butterfat has separated from the buttermilk. This takes 5–8 minutes in a mixer.
Pour off and save the milky liquid (real buttermilk — great for pancakes). Transfer the solid butter to a bowl of ice-cold water.
Knead the butter in the ice water, changing the water 3–4 times until it runs completely clear. Washing removes residual buttermilk which makes the butter spoil faster.
Knead in 0.5 tsp fine sea salt if making salted butter. Shape into a log using cling wrap. Refrigerate for up to 3 weeks or freeze for 3 months.