Let your heavy cream come to room temperature, about 30-60 minutes before making your butter.
Pour the heavy cream into your stand mixer with whisk attachment and splash guard. This makes a mess so be prepared.
Turn the mixer on low and slowly increase speed every few seconds until you're at max. For reference, at 8/10 it took a ton of time to whip up the butter, but at 10/10 it happened very quickly. Speed is important.
After a minute or so, your cream will have become whipped cream. Keep going! It will take 5-10 minutes for the cream to fully separate into butter and buttermilk.
Once this separation occurs, pour off the buttermilk (saving it if you want), and scrape the butter into a ball.
Fill a mixing bowl with COLD water, then transfer the butter ball into it. The rinsing process involves squeezing the butter against the side of the bowl, then re-balling and re-squeezing it.
The water will turn cloudy with excess buttermilk. After a few squeezes, replace the water and keep going, until squeezing the butter no longer clouds the water.
Once done, dry the butter by squeezing moisture out of it with paper towels or cheesecloth.
Finally, add in your salt or mix-ins. You can do this in the mixer again, or just mix it together with a spoon. When you're done, transfer the butter to an airtight container or wrap it in plastic wrap or waxed paper. The more air-tight the better to keep it fresh longer.