Pour your heavy whipping cream into the food processor and puree for 2-4 minutes. Watching for the cream to form hard peaks and start folding on itself.
Then drop 1/8 cup of ice cold water and 1 crushed ice cube through the hole and puree for another 1-2 minutes. If you'd like to make your butter salted now is the time to add the pinch of salt. You should start seeing it hardening even more. You can now check to see if there's any liquid to pour off, that is buttermilk which you can save for another recipe.
Then add 2 more crushed ice cubes and then continue processing for another minute or two until it looks crumbly.
Put the butter in a strainer and gently squeeze out the buttermilk. I've done this without a strainer as well, you just have to slowly and carefully squeeze it out.
Now wrap it wax paper, saran wrap or place it into a container with a lid and refrigerate for at least 15 minutes.
*This recipe is for a 7 cup food processor. If you've got a smaller one, just adjust accordingly, add the heavy whipping cream just below the fill line and only add 1/8 cup of ice water.