Pour the double cream into a cold, sterilised mixing bowl. If it's homogenised, it will still whip, but not as well. If you're using raw cream and want a more traditional taste, leave it to ripen in a cool place, where the temperature is about 8°C (46°F), for up to 48 hours. Whisk the cream at a medium speed in a food mixer until it is thick. First it will be softly whipped, then stiffly whipped. Continue until the whipped cream collapses and separates into butterfat globules. The buttermilk will separate from the butter and slosh around the bowl.Photograph: Peter Cassidy