>> source: The King Arthur Baking Company All-Purpose Baker's Companion

Mix 1 cup of @whole milk{1%cup} with 1 tablespoon @vinegar or lemon juice{1%Tbsp} and let it sit for 10 to ~{15%minutes}. [-The acidity in buttermilk tenderizes the gluten (protein) in flour, plus it works with baking soda better than plain milk by both neutralizing the flavor of the soda at the same time it is activating it, providing wonderful leavening.-]