Beat the double cream with a balloon whisk or electric whisk unti it thickens

Continue beating with a spatula until the mixture separates into solids and liquid

Squeeze the liquid out of the solids using your hands, then transfer the solids to a bowl of cold water

Continue to squeeze the butter in the water, washing out all the butter milk

Finally squeeze out any excess liquid, flavour if you wish, and roll into a cylinder

Wrap in greaseproof paper and store in the fridge for up to 1 week