Whip the cream

• Pour the cold double cream into a large mixing bowl.
• Whisk steadily until the cream thickens and starts to split into butter and buttermilk.

Add the lemon zest

• Grate the lemon zest into the bowl while the cream is splitting.
• Keep whisking until the butter clumps together and the buttermilk separates.

Drain the buttermilk

• Pour off the buttermilk through a colander and keep it for another use.
• Press the butter to release as much liquid as possible.

Season with lemon and soy

• Squeeze the lemon juice over the butter and work it in.
• Add the soy sauce and flaky sea salt, then mix until smooth and evenly seasoned.

Shape and chill

• Pack the butter into a small dish or shape it into a log.
• Chill until firm, then stamp or score the top if you like.