Bowl of ice water for washing the butter
Add the cream and the salt to your stand mixer bowl.
For unsalted butter, simply omit the salt from the recipe.
Using the whisk attachment, whisk on high speed until the cream becomes thick.
Stop the machine, and scrape down the sides of the bowl, using a spatula, you may have to do these 3 or 4 times during the process.

Strat the machine away again, on the same high speed.
Whisk for a further 2 minutes, stop and scrape down the sides again.
Set your machine away again on high speed.
Once your cream starts to resemble scrambled eggs, slow the machine down.

Watch closely for your butter to start separating from the liquid (buttermilk).
Once your butter fat has formed inside the wire of the whisk and completely separated from the liquid, stop the machine, (this should have taken 6 to 8 minutes from the start).

Rinse the butter (whilst still attached to the whisk) under the cold tap, to rinse away the bulk of the liquid buttermilk.
Get a large bowl of clean cold water and add a jug of ice cubes.
Wearing a pair of disposable latex gloves, pry the butter out of the whisk and into the ice water.
Your butter will be floating in the water, gather it all together and press it into a block, dipping it in the water continuously.