Buttermilk began as the slightly soured by-product of freshly churned butter, hence the name. Today it’s made by adding a culture to low-fat milk. It’s similar to yogurt, with a tangy flavor and a thick texture, even though it’s low in fat. The acid in buttermilk activates the baking soda in biscuits, yielding lighter, flakier results, and tenderizes meat (mmm, fried chicken). The smallest container sold is almost always more than you need for a recipe. So here you go—six ways to use it up.