Wash canning jars, lids, and bands in hot soapy water. Place jars in simmering water to keep warm. Bring a large pot of water to a boil and prepare an ice water bath.
Score an X on the bottom of each peach, then blanch in boiling water for 30-45 seconds. Transfer immediately to the ice bath, then peel, pit, and chop the peaches.
Mash the chopped peaches with a potato masher to your preferred texture. Stir in the fresh lemon juice.
In a heavy bottomed pot, combine the mashed peaches, lemon juice, pectin, and optional butter. Bring to a full rolling boil over high heat, stirring constantly.
Boil hard for exactly 1 minute, stirring constantly. Add all the sugar at once, stir until dissolved, and return to a full rolling boil.
Boil hard for exactly 1 minute, then remove from heat immediately. Skim off any foam from the surface.
Working quickly, ladle hot jam into warm sterilized jars using a funnel, leaving 1/4 inch headspace. Wipe rims clean, center lids, and screw bands fingertip tight.
Process filled jars in a boiling water bath canner for 10 minutes. Remove with a jar lifter and let cool undisturbed for 12-24 hours before checking seals.