I often use plant milk (almond, cashew or soy) and vinegar in lieu, but here's the secret: use a really good tasting vinegar such as balsamic. The acid is important, but so is the flavor. Also sometimes I add a little olive oil to replace some of the fat. Works just fine in the simplest and best use for buttermilk: mashed potatoes, which is all about tangy flavor, and not about rise. Lemon juice is brighter, and better for sweet things like buttermilk pie.

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