1 gallon fresh apple cider
1 1.75-oz. packet powdered pectin
1/2 tsp butter
1/2 to 1 cup granulated sugar (optional)
Pour the apple cider into a stockpot that holds at least 5 quarts of liquid. Bring to a boil over medium heat and then reduce the heat as necessary to keep the apple cider at a good simmer, bubbling happily and not sluggishly. After about 2 to 2 1/2 hours, the cider will have reduced to a little more than a quart, with about 5-6 cups of cider left in the pot.
(While the cider simmers and reduces, wash and rinse 4 8-oz. mason jars, rings, and fresh lids. Keep the jars and rings in a 200˚F oven until you’re ready to use them, and the lids in a pot of barely simmering water until then. If you’re water bath canning your jelly, you’ll probably want to get your canning pot ready and on for a boil now too.)
Whisk the pectin and sugar together in a small bowl, if using any sugar.
Transfer the cider to a smaller 2-quart saucepan, if desired, add butter, and whisk the pectin (and sugar) into the cider. Simmer the cider for 5 minutes more, letting the pectin activate and thicken the cider into a syrupy jelly.
Carefully fill the warm jars with jelly, leaving 1/4 inch headspace, and water bath can for 10 minutes at a rolling boil if desired. If you’re just making refrigerator jelly, seal the jars and leave them on a cooling rack until no longer hot to the touch. Transfer the jars to the refrigerator and allow to chill for 24 hours before using.
Water bath-canned jelly will keep, sealed, at room temperature for up to a year. Refrigerator jelly will keep, sealed, for up to 2 months in the refrigerator. Once jelly has been opened, store in the fridge for up to 3 weeks.