Pit the cherries if necessary, then chop them up. Crush the remains thoroughly, 1 cup at a time. If using frozen berries, use both liquid and solids; they all were part of the original fresh berry. 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 canner.