In a clean stand mixer fitted with the whisk attachment, pour in the cream.
Mix cream on low speed, slowly increasing to medium, until stiff peaks form.
Keep bring the speed up to high and keep mixing. The cream will start to look curdled. At this point, place some dish towels over the top of the mixing bowl to minimize splashing.
Keep mixing until all the fat is together (and will likely be stuck on the whisk attachment) and separated from a white milky liquid.
Strain off the white liquid (buttermilk) if you want to keep it.
Remove the butter from the whisk and place run a bowl full of ice water.
Knead and massage the butter in the ice water. Once the water gets cloudy, dump it out. Repeat until the water is clear. That's how you know you washed out all the buttermilk and your butter will last longer in the fridge.
Remove your butter from the ice water and sprinkle salt onto (optional. Knead it in.
Place your butter in a airtight container and store in fridge for 1-2 weeks.
