Place the double cream in a food processor, and blend on a high speed until the cream curdles, about 10 minutes.
If you do not have a food processor, the cream can be whisked until curdled with an electric mixer, or even, for a more physical activity, in a tightly closed jar.
Once the cream is curdled, pour into a sieve to get rid of the butter milk. Return the butter to the food processor, cover with ice-cold water, and blend for a further 5 minutes. Discard the milky water.
Repeat the process until most of the buttermilk is washed out: the better the butter is washed, the longer it will keep.
Once the buttermilk is washed out, knead the butter on a clean tea towel. If desired, add Maldon salt, or any other flaky sea salt, at this point. Fine salt can be used instead, however, finer salts tend to disolve more quickly, resulting in a butter that is evenly salted as opposed to one that has the occasional salty burst.
Wrap the butter in baking parchment or in an airtight container, and refrigerate until needed.
«For a more traditonal style of butter, try using creme fraiche: the butter will have a slight tang, just like cultured butter, and the resulting buttermilk can be used in any recipe calling for it.»