Method

Whisk the cream at a medium speed in a food mixer until it is thick. First it will be softly whipped, then stiffly whipped. Continue until the whipped cream collapses and separates into butterfat globules. The buttermilk will separate from the butter and slosh around the bowl.
Turn the mixture into a cold, spotlessly clean sieve and drain well. The butter remains in the sieve while the buttermilk drains into the bowl (it can be used to make soda bread).
Put the butter back into a clean bowl and beat with the whisk for a further 30 seconds to 1 minute to expel more buttermilk. Remove and sieve as before.
Fill the bowl containing the butter with very cold water. Use hands to knead the butter to force out as much buttermilk as possible. This is important, as any buttermilk left in the butter will sour and the butter will go off quickly. If you handle the butter too much with warm hands, it will liquefy. Drain the water, cover and wash twice more, until the water is totally clear.
Add salt to taste and store in the fridge or freeze.