Smooth and tangy, buttermilk only sounds decadent. In reality it is just the milk that's left behind after cream is churned into butter. These days, most store-bought buttermilk is made from milk treated (or "cultured") with lactic acid bacteria. The result: a delicious, multi-purpose ingredient that can tenderize meat in marinades and give baked goods a lofty rise. Here are 12 ways to use it.