Pour the cold heavy cream into the bowl of a stand mixer fitted with the whisk attachment. Mix on medium speed until the cream becomes whipped cream, then continue mixing until the butter separates from the buttermilk, about 10–15 minutes.

Place a fine mesh sieve over a bowl and pour the mixture through the sieve. Reserve the fresh buttermilk for baking if desired.

Transfer the butter to a bowl of ice-cold water. Gently press and fold the butter with a wooden spoon or your hands. Drain the cloudy water and repeat with fresh cold water until the water runs mostly clear, about 3–5 washes.

Press the butter firmly against the side of the bowl or wrap it in a clean tea towel and gently squeeze out any remaining water.

Mix in the salt, if using. Leave the butter unsalted for baking or future flavoured butter recipes.