Combine. 1/4 cup yoghurt and 400ml/14oz of double cream (min 35% fat) in a large bowl. Whisk together then cover with a clean cloth and leave to sit on the bench for 24 hours.
Mix. Once the cream is fermented, pour it into the bowl of a mixer and mix on high until it separates into a solid (butter) and a liquid (buttermilk). This will take roughly 3 - 4 minutes.
Strain. Line a bowl with muslin then pour in the butter and buttermilk. Squeeze to strain out the buttermilk and keep this in a jar in the fridge to use later.
Rinse. Form the butter into a ball and dunk it into iced water several times. Squeeze as you do this to rinse the butter and remove any residual buttermilk. This will ensure the butter won't spoil.
Shape with butter pats and store in a butter dish, press into a small dish, or roll it into a log in baking paper then store in the fridge. Sprinkle the top with salt before storing (optional).
For best flavour, use the buttermilk within a couple of days and the butter within a month. Freezing them is also an option to ensure they don't go to waste.
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