Place 1 (48-ounce) bottle of prune juice, preferably with pulp, in a 5- quart non-stick pot over high heat and bring to a boil. Cook until the liquid is reduced to 1.5 cups, about 50 to 55 minutes. Let cool, then place in a covered container and refrigerate.
Remove syrup from the refrigerator as you start your preparation, so it will have time to come to room temperature. The syrup will be thinner and easier to pour and measure.