Cut the plums into halves or quarters, removing pits.
Extract Juice:
Place the cut plums into a large pot and add just enough water to cover them.
Bring to a boil, then reduce the heat and simmer until the plums are soft (about 10-15 minutes).
Use a jelly bag or cheesecloth to strain the juice. Allow the juice to drip through into a bowl (do not squeeze the bag to keep the juice clear). Measure out 4 cups of plum juice.
Make the Jelly:
In a large pot, combine the plum juice and powdered pectin.
Bring to a rolling boil over high heat, stirring constantly.
Add the sugar all at once and return to a full rolling boil. Boil hard for 1 minute, stirring constantly.
If using, add the butter to reduce foaming.
Test for Doneness:
Remove from heat and test the jelly by using the spoon test: dip a cold metal spoon into the jelly and lift it out sideways. The jelly should sheet off the spoon rather than run off in a stream.
Alternatively, use a candy thermometer to ensure the jelly reaches 220°F (104°C).
Prepare Jars:
Sterilize canning jars and lids by boiling them in water for 10 minutes.
Use a canning funnel to pour the hot jelly into the sterilized jars, leaving 1/4 inch of headspace.
Wipe the rims of the jars with a clean, damp cloth and apply the lids and bands.
Process the Jars:
Process the jars in a boiling water bath for 5 minutes (adjust time for altitude if necessary).
Remove the jars from the water bath and let cool on a towel or rack. You should hear a popping sound as the jars seal.
Store and Enjoy:
Once cool, check the seals by pressing the center of each lid. If the lid doesn’t flex up and down, it’s sealed.
Store in a cool, dark place. Refrigerate after opening.
