This is for jbermo, who asked a question on the Los Angeles board (http://chowhound.chow.com/topics/813293) about how to make your own cultured milk (buttermilk).
All cultured products can be done the same way—and while you can go to a health foods store and buy starter, honestly, I normally start with active-culture plain yoghurt or regular Knudsen's (or equivalent) buttermilk.
The instructions are the same for all of the products below. Take a quart of liquid dairy product and add half a cup of liquid active-culture buttermilk (the fresher the better, in this case, in case of bacterial die-off). Mix them together in a scrupulously clean glass bowl (you can use non-reactive metal too, but don't use plastic), cover tightly with cling film, and set out for 24 hours on the counter. Not in the fridge, on the counter.
After that time, you will have cultured dairy product. Keep it in the fridge and use it within a week.
If you use whole milk, you will have thin "buttermilk" (the quotes are there because technically buttermilk is the leftover liquid from the buttermaking process).
If you use half-and-half, you will have thick "buttermilk".
If you use heavy cream, you will have cultured cream, which has a host of uses. The French call it crème fraîche; Mexicans call it crema agria. It will be a little bit thin; you can thicken it by draining it through cheesecloth, coffee filters, or an old, clean, white t-shirt.
If you whip crème fraîche past the point of whipped cream until it "breaks" (put a towel over the mixer or you will have liquid EVERYWHERE), you will have Normandy-style cultured butter, akin to Isigny Ste.-Mère, except at a third of the price of buying in the markets. (After you gather the butter and let it drain, wash the butter under gently running water to get pockets of buttermilk out and then re-drain without saving the "water" liquid; you can knead a pinch or two of fine salt in at this point if you want to simulate the half-salted butter of northwestern France.)
The liquid left over from making cultured butter is real buttermilk; it will be extremely thin but will be acidic enough for your baking needs—or use it to start your next batch of cultured dairy.
You can thicken the products either by draining them a bit, or by increasing the amount of buttermilk/yoghurt in the original mixture; if it comes out too thick, next time use less buttermilk/yoghurt.
If you don't want to start from a commercial preparation, you can buy yoghurt starter in most co-ops and health stores. The proportions to start with are 4 teaspoons of starter per quart of dairy.