Pour the cold cream into a stand mixer bowl fitted with the whisk attachment. Keep the bowl no more than about halfway full because the cream expands and may splash.

Start mixing on low speed, then increase to medium-high. Continue past the whipped-cream stage until the mixture turns grainy, separates into yellow butter clumps, and releases thin white buttermilk.

Turn off the mixer and carefully drain the buttermilk into a container. Reserve it for pancakes, biscuits, or other baking.

Add ice-cold water to the butter and gently press or knead it with a spatula or clean hands. Drain the cloudy water and repeat with fresh cold water until nearly clear.

Press the butter firmly to remove as much liquid as possible, then mix in the salt if making salted butter. Shape, cover, and refrigerate.