Boiled cider is a highly concentrated and intensely flavored apple cider. If you do not have it handy, you can substitute additional buttermilk. Or, to make your own boiled cider: Add 1/2 gallon of apple cider to a large (non-reactive) Dutch oven or pot. Over medium-high heat, bring the cider to a boil. Reduce heat to medium-low and let cook, stirring occasionally, for 4-5 hours, or until the cider has reduced to about 1 cup and has a thick, syrupy consistency. Transfer to a small jar and store in the refrigerator.