Crush berries thoroughly, 1 cup at a time. If using frozen berries, use both liquied and solids; they were all part of the original fresh berry. (Sieve 1/2 of the pulp to remove some of the seeds if desired. You can sieve it all if preparing from those with dental problems. Removing seeds causes waste, so be sure you have enough berries.).
Make sure your canning equipment is scrupulously clean, and place 7 pint or half-pint jars on a rack in a boiling water canner. Fill the canner with water, adding a shot of vinegar to prevent mineral buildup, and bring to a boil while you prepare the jam. Allow it to boil for 10 minutes to sterilize jars, then turn down the heat to keep the jars hot. Remember to keep a kettle of hot water handy, and a saucepan of boiling water for your lids and rings.
Measure 6 cups of crushed fruit into a 6-8 quart heavy nonreactive saucepan. Break the chocolate squares into smaller pieces and add them to saucepan.
Measure sugar into a separate bowl. Mix 1/4 cup of the measured sugar with the pectin powder in another small bowl. Stir pectin-sugar mixture into fruit in a saucepan. Add butter. Bring quickly to a full rolling boil. Boil for EXACTLY 1 MINUTE, stirring constantly.
Remove pan from heat. Skim off any foam and ladle the jam into hot sterilized pint or half-pint canning jars, leaving 1/4 inch headspace.
Process for 10 minutes in a boiling water canner.
Remove the jars to a towel/newspaper lined surface, invert for 10 minutes, and allow to cool. Label the jars and store in a cool dry place for up to 1 year.