I should qualify the previous post by noting that real buttermilk is a thing that isn’t around anymore. People don’t much churn their own butter, and changes in commercial butter-making techniques have eliminated real buttermilk from the food chain. I find that a shame, because my grandfather drank it (he also liked beef consommé over ice, the weirdo), as did the winners of the Indy 500 once upon a time.
Today’s buttermilk is properly called cultured buttermilk and is made by adding Streptococci bateria to skim or 2% milk to give it thickness and a tangy taste. But while it may share many of the same surface properties of the real thing, it’s chemically very different, which is why, if you have some truly old recipes that call for real buttermilk, they likely won’t work anymore.
But the key thing to remember about buttermilk, whatever the stripe, is that it isn’t actually buttery. Traditionally, it’s milk from which all the butter had been removed, so while it may be thick, it is by no means rich, being either nearly or entirely fat-free.