Whisk the crème fraiche vigorously in a bowl or a food mixer, or shake it in a large jar with a screw top lid. It will get thicker and thicker and eventually start to separate and go grainy and sloppy. Keep whisking and it will noticeably separate into fat and liquid. Line a bowl with a j cloth and pour the mixture into it. Gather up the sides of the cloth and twist to form a bag. Keep twisting and squeezing the bag over the bowl - stop when you see the butter just start to come through the holes. Tip the butter into a clean bowl and pour in some very cold water. Squeeze the butter a little in the water – this will firm it up and wash off the remaining buttermilk – it will keep better that way. Drain it, and it is ready.
If you want to make salted butter, add salt to taste and work it in using two spoons or spatulas.
Keep the buttermilk and use it to make scones or soda bread, or use it in a smoothie, or a soup. Or just drink it.
«500g crème fraiche will give you about 250g butter and 250g buttermilk…of course, you can make any amount you like.»