Make homemade butter from cold heavy cream using a hand mixer. Beat until the butter separates, rinse and shape it, then refrigerate for a firm, sliceable finish.
Pour 600 ml heavy cream, cold, into a large mixing bowl. Beat with a hand mixer for about 10 minutes, watching as the cream becomes whipped and then grainy.
Just before the liquid begins separating, stop the mixer. Secure plastic wrap over the bowl, leaving an opening for the beaters and keeping the wrap clear of the moving parts. Continue beating until the yellow butter solids separate completely from the liquid.
Stop the mixer and remove the plastic wrap. Pour the liquid buttermilk into a separate container, then gather the butter together.
Place the butter in cold water and press gently to release the remaining buttermilk. Drain the cloudy water and replace it with fresh cold water. Repeat until the water stays clear.
Drain the butter and gently press out any remaining water. Transfer it to parchment paper and shape into a block. Sprinkle with 1 pinch sea salt, if using.
Wrap securely and refrigerate until firm, allowing approximately 2 hours. The chilling time will vary with the size and shape of the block.
Notes
Cream choice: Use heavy cream or heavy whipping cream, rather than milk or half-and-half. Unsalted option: Leave out the sea salt when plain butter is preferred. Salt sprinkled over the block seasons the surface. Leftover liquid: The liquid from this process is uncultured buttermilk. It is different from cultured store-bought buttermilk and should not automatically replace it in baking. Storage: Keep the wrapped butter in an airtight container in the refrigerator and use within one week for best quality.