Pour the strawberries into a large pot. Measure the sugar into a separate bowl and set aside. Stir the package of pectin into the strawberries and add the butter.
Stirring frequently, bring the strawberry mixture to a full rolling boil (a boil that does not stop bubbling when stirred), and then add the sugar. Stir to fully incorporate.
Return the mixture to a full, rolling boil and boil for exactly 1 minute, stirring constantly.
Remove the pot from the heat.
Skim any foam from the surface.
Ladle quickly into clean jars. (I like to use a wide funnel.)
Wipe the rims of the jars with a clean cloth.
Place the heated lids (lids that have been placed in simmering water for at least a minute) on the jars and tighten the bands.
Place the jars in a canner and process (gently boil) for 10 minutes. The water should cover the jars by an inch or two–add additional boiling water, if necessary. Carefully, remove the jars from the canner.
Cool completely before storing. Make certain all lids have sealed properly before storing. If the lid springs back when pressed in the middle, it is not sealed and should be stored in the refrigerator.
