When you make butter with soured cream, the liquid that separates out is buttermilk. If you try it, make sure you don't get ultra-pasteurized cream. I buy Clover pasteuized heavy whipping cream. Then leave the cream out at room temp for a few days so it begins to sour. Pour it into a mixing bowl and whip it until you're well past whipped cream. Keep mixing on high until a white-ish liquid separates out from the fat. The fat is butter; the liquid is buttermilk. If you haven't soured the cream you can still make butter, but the liquid won't have the acids it needs to be buttermilk you can use in baking recipes.