Wash and hull strawberries. Mash thoroughly (do not purée). Measure 5 cups prepared fruit into a large pot.

Add the pectin (and butter, if using). Stir well.

Bring to a full rolling boil over high heat, stirring constantly.

Add all the sugar at once. Stir until dissolved.

Return to a full rolling boil that cannot be stirred down; boil exactly 1 minute, stirring constantly.

Remove from heat. Skim off any foam.

Immediately ladle hot jam into hot jars, leaving ¼-inch headspace. Wipe rims, apply lids and bands fingertip-tight.

Process jars 10 minutes in a boiling-water canner (add 1 minute per 1,000 ft elevation).

Cool 12–24 h without disturbing. Check seals, label, and store.