For each cup of buttermilk needed, to a one cup measure or larger, add one tablespoon of fresh lemon juice or white vinegar, then top off with whole milk. Stir and let sit to "sour" for about 15 minutes. Stir again and proceed with recipe.
It's not a perfect substitute, but it'll generally work in a pinch!
In some recipes, 3/4 cup of plain, unflavored yogurt and 1/4 cup of whole milk will also work as a substitute for buttermilk.
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