1 cup heavy cream
Place bowl and blade of a food processor into the freezer for 4 to 5 hours.
Refit the bowl and blade, turn on the food processor, and pour the cream through the feed tube. Process for about 10 minutes, until butter has hardened around the blade, leaving the liquid that has separated from the cream in the bottom of the bowl.
Transfer the butter to a sieve and shake 2 or 3 times over the sink to allow any excess liquid to drain. Put the butter in a bowl, cover, and chill for at least 1 hour before using.
Store it in the refrigerator for up to 1 week.
Makes 2/3 to 3/4 cup.