Combine the root beer soda and lemon juice in a large stock pot. Sprinkle the powdered pectin on top of the juice mixture and use a whisk to mix it together. Stirring constantly, heat until boiling. Boil for one minute.

Add the butter and return to a boil. Add the sugar to the pot all at one time (measure it and have it ready ahead of time). Stir until sugar is dissolved. Return to a boil and boil for one minute.

Remove the pan from the heat. Remove any foam with a metal spoon. Ladle the jelly into hot sterilized canning jars, leaving 1/4 inch head space, and process in boiling water canner for 10 minutes.

Remove jars from canner and set the jars on the counter on a kitchen towel to cool.