Answer: Combined 1 cup milk with 1 tablespoon lemon juice. Let sit for two minutes and use the amount the recipe calls for.
Why: Buttermilk's name is deceiving as it contains less fat then regular whole milk. The process of making buttermilk is similar to making yogurt. As it goes through a fermenting process the acidity level of the milk increases. In baking, many recipes that call for buttermilk often included baking powder. Baking Powder works as a double leavening ingredient and will react first upon contact with a liquid and a second upon contact with an acid. By increasing the acidity of milk with lemon juice you are triggering the second reaction to acidifying agent. 'Double-Acting Baking Powder' is baking powder the has a third reaction when it is exposed to heat.