Return the butter to the food processor and add a small amount of ice cold water (use about half the amount of water as your original amount of cream). It must be cold water, but cannot be actual ice. Turn food processor on and allow it to blitz for 30-60 seconds. Pour out water, ensuring it is thoroughly drained. This processor will harden up the butter, but is also essential for rinsing off the remainder of the buttermilk, therefore extending the shelf life of the butter.