Add the heavy cream to an electric mixer. Start whipping the cream on low speed until it thickens slightly.
Gradually increase the mixer speed to medium-high and continue to whip for 5-7 minutes, scraping down the sides every now and then. Continue mixing past the whipped cream phase, and you'll see the cream begin to break into butter (fat molecules) and liquid (buttermilk).
Remove the butter from the mixer (save the buttermilk for use later!) and squeeze out any excess moisture by repeatedly squeezing the butter through your hands.
Quickly rinse the butter under ice cold water and add salt to taste.
Store in either an air-tight container or rolled up in parchment paper. Keep it in the fridge and use within 10-14 days.