To make 230ml of buttermilk, beat 700ml double cream fast until the sound changes and it properly separates into curds of butter and watery buttermilk. (Those of you who've over-whipped cream may not have known you were just moments away from making homemade butter.)
Sieve the curds out over a big bowl, squash them down a bit to get all the milk out, then plop the curds into very cold water.
With a firm hand, squish the curds together underwater to make a rough ball of butter. Place on a piece of grease-proof paper and roll it up.
You now have a luscious pack of your own hand-made butter to savour, melted over a slice of warm soda bread. You also have, in the big bowl, the acidic buttermilk, perfect for making our soda bread recipe.