Crush berries thoroughly, 1 cup at a time. If using frozen berries, use both liquid and solids; they all were part of the original fresh berry.
(Sieve 1/2 of the pulp to remove some seeds if desired. You can sieve it all if preparing for those with dental problems. Removing seeds causes waste, so be sure you have enough berries.)
Measure 6 cups of crushed fruit into 6- or 8-quart heavy saucepan. Break the chocolate squares into smaller pieces and add them to saucepan.
Measure sugar into separate bowl.
Mix 1/4 cup sugar from measured amount with pectin in small bowl. Stir pectin-sugar mixture into fruit in saucepan. Add butter. Bring quickly to full rolling boil and boil exactly 1 minute, stirring constantly.
Remove from heat. Skim foam and ladle into pint or half-pint jars, leaving 1/4-inch headspace and process 10 minutes in boiling water.Servings: