Put your cream into a blender, food processor, stand mixer or mixing bowl.
Mix until the cream breaks and separates. This usually takes less than 10 minutes. You will see a liquid (buttermilk) in the bowl or mixer and the butter will begin to thicken.
After it separates, pour icy water into the bowl with the the butter & buttermilk. This will help to separate out more buttermilk.
Place a strainer or a piece of cheesecloth over a large bowl and pour the butter, butter- milk, and water over it.
Squeeze the butter with your hands, with a clean towel, or with the back of a spoon to squeeze out more buttermilk. You don't want wet butter so this is important.
Stir in salt at this point if you wish.
You can use this butter immediately or you can place it in the refrigerator for it to harden. It should last covered in the fridge for 1-2 weeks.