Remove heavy cream from fridge and let stand at room temperature for 30-60 minutes to encourage separation.
Pour heavy cream into stand mixer bowl, fitted with whisk attachment. Place bowl guard/pouring shield on bowl to prevent splashing.
Gradually increase mixer from lowest to highest setting. This should take 20-30 seconds. After about a minute or so it will look like like whipped cream. Keep going at full speed — after about 3 minutes it will separate. The butter will collect inside the whisk and the buttermilk splash around in the bottom of the bowl.
Spoon any lumps of butter out of the buttermilk and smush them onto the butter in the whisk. Pour the buttermilk into a separate container and set aside. You can use it for baking, drink it, or freeze it for later.
Fill a bowl with 3 cups of water and a few ice cubes. Use a spatula to knock the butter off the whisk into the water. Swirl the butter around, then start gathering the pieces together on a paper towel. Change the ice water a few times as needed until it runs clear. Then smush and fold the butter firmly against the walls of the bowl to press out any additional buttermilk.
Wrap the butter in several layers of cheese cloth or two layers of paper towels and squeeze to remove any additional moisture. Flatten the butter into a disc between two paper towels, then fold back into a ball and repeat, until the paper towels aren't picking up any moisture.
Place the butter back in the mixer bowl and beat on medium with the paddle attachment. Add salt or any desired herbs to taste and beat again to combine.
Wrap butter well in an airtight container and chill.
If you don't have a bowl guard or shield for your mixer you can stretch plastic wrap over the bowl and cut a hole in the middle for the whisk attachment to fit through.