Buttermilk Powder Is Not an Option. It Is a Necessity.
Instead of worrying about how you'll use up that carton, pick up the powder instead.
I always end up with too much or too little buttermilk. I don't know why that's my specific, mildly annoying but pretty inconsequential curse, or what baking fairy decided on the day I was born that biscuits were just always going to come with a side of hassle, but it's true: I either have half-an-inch left at the bottom of the carton and have to make a quick milk-and-vinegar substitute to make enough for my recipe, or I'll forget about three-quarters of a carton and dig it out a month later to make pancakes, only to find a fine layer of blue mold growing on the inside.
Yes, I'm aware you can freeze buttermilk, and use it in all kinds of creative ways, but despite that knowledge, it remained a regular casualty of my overcrowded fridge—until I discovered the wonder that is buttermilk powder.
Buttermilk powder is what it sounds like: buttermilk that has been dehydrated until it becomes a fine powder, similar in texture to flour, or regular milk powder. But unlike regular buttermilk, it lasts practically forever tucked into the back of your fridge. You simply mix a tablespoon of buttermilk powder into a cup of water, and use it as you would the liquid kind.
I was at first convinced that it was convenient, as a cheat, but not actually as good as buying a quart of buttermilk from the fridge. But to my surprise, biscuits, cakes, pancakes, and other baked goods tasted just as good with the powder. I'm sure if you could get your hands on fresh-from-the-farm true buttermilk it might be better, but powdered buttermilk is great in all the usual things I use it for, including marinades. I even make crème fraîche with it—I just mix a tablespoon of buttermilk powder into a cup of heavy cream and let it sit out for a day. Voila, homemade, cheaper-than-store bought crème fraîche. Never be without—or overwhelmed by—buttermilk again.