Making butter now involves quite a bit less work than it did in olden days; a few minutes in the food processor replace hours of churning. But if you would like a workout (or an afternoon of activity for small and enthusiastic children) put the cream in a tightly sealed Mason jar and shake away.
about 1/2 cup
1 cup heavy whipping cream
1 tablespoon sour cream
1/8 teaspoon salt (optional)
Whir the cream and sour cream in a food processor for about five minutes. (Alternately: shake energetically in a Mason jar for about 15-30 minutes.) After the cream becomes thick and clotted like whipped cream it will suddenly start spattering against the top again as it clumps into yellow butter. Whip until the butter has formed a solid yet grainy mass.
Put a small mesh strainer over a bowl and pour everything into it. Refrigerate the liquid; this is buttermilk and it will last for several days. Use it for pancakes, or Pecan Cake, or Meatballs with Buttermilk Gravy.
Put the butter in a small bowl and rinse under very cold water until the water runs clear. You want every bit of the buttermilk removed; any left clinging to the butter will cause it to get sour and bad overnight.
When the water runs completely clear squeeze the butter inside a clean paper towel or cheesecloth until dry, then turn out into a crock or small bowl. Mash in the salt, if using. Refrigerate and use within a week.