Pour cream into a blender. Cover and blend until cream separates into buttermilk and butter, about 5 minutes. Pour buttermilk into a small bowl, using a spoon to press as much buttermilk out of the butter as possible. Place butter in a large bowl filled with cold water.
Knead butter in the cold water to remove all buttermilk, straining off and replacing the water 7 to 10 times as you work, until it remains clear, about 5 minutes. Pour off the water and knead butter a few more times to squeeze out any remaining water, about 1 minute. Transfer to a plate and knead in salt.