If you don't have buttermilk on hand and need it for a baking recipe, use this easy method to make a DIY buttermilk "soured milk" substitute. This is a great substitution for buttermilk in most baking recipes. However, you'll begin to notice a flavor and texture change in recipes that call for more than 1 cup of buttermilk. For best flavor and texture, I recommend using real buttermilk when you can.