No. Buttermilk is acidic, and any recipe that has been created using it is typically designed to have a specific reaction between the buttermilk (acid) and the leavening agents (usually baking soda). Note: sometimes I do use buttermilk without baking soda, like in my coffee cake and vanilla cake. In these instances it is to enhance the moisture and flavor of the recipe. Regular milk isn’t acidic, which means the reaction won't take place. Your baked goods may not rise properly and can turn out drier, less flavorful, and less tender if you use plain milk where buttermilk is called for.