Add cream and yogurt to Thermomix bowl and heat to 37℃ at speed 2, (this will take around 2 mins. Tip cream mix into a bowl and cover. Leave at room temperature for around 8 hours or overnight to ferment.
After this time, place in the fridge to chill down for a couple of hours.
Insert the butterfly into the Thermomix bowl. Add the cold cream and yogurt mix. Mix until the butter solids have separated from the buttermilk. (This can take 2 - 4 mins at speed 4, depending on how cold your cream mix is). Listen to the Thermomix and keep an eye on things through the hole in the lid. When liquid starts splashing, you know you are done.
When separated, pour the mix through a sieve, keeping the buttermilk (the liquid) to use later on something else. Remove the butterfly from the bowl.
Return solids to the Thermomix bowl with the cold water and salt. RemWash by mixing (without the butterfly) for approx 6 seconds at speed 4.
Drain well through sieve; getting out as much liquid from the butter solids as possible, but not forcing the butter through the holes/
Shape the butter and keep cold in a butter bell, fridge or freezer.
Notes
Please see notes in blog post @collettesthermiekitchen