Place the double cream into a food processor, place the lid on and blitz at high speed until the cream fully splits; creating a ball of butter. Immediately strain through the muslin into a colander over a mixing bowl. Twist the muslin into a ball - not too tight. Unwrap then serve. Can be refrigerated for upto 7 days or frozen for upto 3 months. The liquid is buttermilk. Great for baking or tenderising meat.