Pour the cream into a clean jar with a tight-fitting lid, filling it no more than halfway.

Seal the jar and shake steadily. The cream first turns into whipped cream and the jar goes thick and quiet — keep shaking.

After 5–20 minutes of shaking, the fat suddenly clumps into a pale lump of butter splashing in thin, milky liquid.

Pour off the liquid — that is buttermilk; save it for baking.

Cover the butter with cold water and knead and press it with a spoon to wash out the remaining buttermilk. Drain, and repeat twice more with fresh cold water.

Stir in a pinch of salt if you like. Refrigerate.