Cecilia, it's simple to make a substitute for buttermilk at home: Just measure out a scant cup of milk and add 1 tablespoon of either lemon juice or white vinegar. Let it sit for 5 to 10 minutes. (As I mentioned below, it won't have quite the depth of flavor of real buttermilk, but it'll work well for baking). Good luck! Posie at King Arthur.

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