01 - Pour chilled heavy cream into a stand mixer fitted with the whisk attachment, or use a hand mixer or food processor.
02 - Whip cream on medium-high speed until it thickens and separates into butter and buttermilk, approximately 5 to 7 minutes.
03 - Pour off the liquid buttermilk, reserving if desired for baking or other uses.
04 - Gather the butter into a ball and rinse under cold water, kneading gently until the water runs clear to remove residual buttermilk.
05 - Place butter in a bowl, sprinkle with sea salt, and mix thoroughly to distribute.
06 - If desired, fold in chopped fresh herbs or lemon zest evenly throughout the butter.
07 - Transfer the butter to a container or shape into a log. Chill before serving to set firmness.