When considering alternatives for buttermilk, you require something creamy and something sour. This is why regular milk and an acidic compound make for a great substitute.
To make the substitute, stir a tablespoon of lemon juice or a mild vinegar, like distilled white or apple cider, into a cup of milk and let the mixture sit for a few minutes.
The milk will begin to curdle very slightly. It's usually a bad sign, but when making a buttermilk substitute, it means the milk has been acidified, which is exactly what we want.
Using whole milk or cream will yield the richest result, but lower-fat milk, lactose-free milk, or non-dairy alternatives like soy, oat, almond, or coconut milk will also work.
While a one-for-one swap of buttermilk for this substitute works for baking, they don’t have the same viscosity and won’t yield the same texture in salad dressings and wet dredges.