Prepare your canning station: Sterilize jars and keep them hot, and prepare the lids according to the manufacturer's directions.

Combine the prepared rhubarb and strawberries in a large, heavy-bottomed pot or Dutch oven. Cook over medium heat, stirring occasionally, until the fruit begins to release its juices and softens, about 10-12 minutes.

Increase the heat to high. In a separate small bowl, thoroughly mix the granulated sugar with the entire package of powdered pectin. Sprinkle this sugar-pectin mixture over the hot fruit while stirring constantly to prevent the pectin from clumping.

Bring the mixture to a full, rolling boil that cannot be stirred down. Once boiling, add the optional butter to dissipate foam. Boil hard for exactly 1 minute, stirring constantly. Remove from heat immediately.

Carefully skim off any surface foam using a long-handled spoon. Ladle the hot jam into the sterilized jars, leaving 1/4 inch headspace. Wipe rims clean and affix lids and bands fingertip tight.

Process the jars in a boiling water canner for 10 minutes (adjusting time for altitude if necessary). Turn off the heat, let the jars rest in the hot water for 5 minutes, then remove to a towel-lined counter to cool completely for 12-24 hours before checking seals.