Wash and sterilize your jars and lids in boiling water, then let them air dry.
In a large pot, combine the mashed strawberries, lemon juice, and fruit pectin over medium heat. Stir well to combine.
Bring the mixture to a boil, stirring frequently. Once it reaches a rolling boil, add the granulated sugar all at once.
Stir the mixture continuously and bring it back to a rolling boil for 1 minute.
If using, add the butter to reduce foaming and stir well.
Remove the pot from heat, and skim off any foam that forms on the surface.
Using a ladle and funnel (if available), fill your sterilized jars with the hot jam, leaving about ¼ inch of headspace.
Wipe the rims of the jars to ensure a good seal, then place the sterilized lids on top and screw them on tightly.
Let the jars cool completely at room temperature, then store in the refrigerator or process them in a water bath for longer shelf life.