Prepare jars for canning: Wash five half-pint jars, bands and lids. Place the jars in a water-bath canner, fill the canner three-quarters with water and heat until boiling. Pour hot water into a bowl; add the jar lids and screw bands.
Juice the berries: In a large saucepan over medium heat, warm berries (with stems removed) and crush them thoroughly. Stir occasionally until the juice and berries reach a boil. Then reduce heat and simmer for 15 minutes. Place a fine mesh strainer over a large bowl. Pour mixture into the strainer lightly press berries to get all of the juice out of them; discard berries in a sieve.
Make the jelly: Measure 3 cups of juice from the bowl and place it in a large sauce pot. Add lemon juice. Place over medium heat and add pectin. As the mixture comes to a boil, it will foam. Add butter to reduce foaming of the mixture. Then turn heat to high until it is at a rolling boil.
Once mixture continues to boil even when stirred, start to add the sugar. While stirring constantly add sugar two cups at a time. Continue stirring until mixture returns to a boil, boil for exactly one minute. Remove from heat.
Carefully remove the jars, lids and screw bands to a wire rack and pat dry. With a plastic ladle and canning funnel, fill the jars with the hot jelly mixture, leaving a half-inch of headspace. Wipe the jar rims to remove any drips. Top with lids; screw on bands. Process the bars in a boiling water bath for 5 minutes. Water level should cover the jars by at least one and a half inches. Add more boiling water if necessary.
Allow jars to cool completely at room temperature on a trivet or towel, about 24 hours. You’ll know their seals are tight when the lids start to pop.
Test Kitchen tip: If you have extra jelly or a jar didn’t seal properly, stash it in the fridge and eat it first! It’ll last for months there. On the shelf, sealed jars will last a year.
Use elderberry jelly on anything from breakfast to dessert. It's delicious on toast and on a cracker with goat cheese.