Insert the butterfly whisk.Add the cream and mix for 1-4 min/speed 4 until it separates. The cream's timing to turn into butter will vary on the freshness of the cream and how cold it is. It's ready when the sound of the blades change, and if you have the TM31, the lid will start to 'dance' (see photos in recipe post).
Remove the butterfly whisk. Using the simmering basket, strain the buttermilk into a bowl, so all you're left with is butter. You can use the buttermilk for waffles, scones, pancakes etc.
Place the butter back into the Thermomix bowl (without the butterfly whisk), add 500g of ice-cold water and mix for 5 sec/speed 4 to rinse it. Strain again into a bowl but this time discard the liquid. Rinse the butter three times until the water runs clear. Add ½ a teaspoon of sea salt for salted butter after the final washing and mix for a few seconds to incorporate.
Tip the butter into the simmering basket over a bowl, squeeze out the excess water with your hands and knead into a ball or sausage shape. (see recipe post for step-by-step photos).
Wrap the butter in cling film and store it in the fridge. Unsalted butter should be consumed in a few days, but with the addition of salt, it will keep in the fridge for two to three weeks.