Pick-over but do not wash raspberries. Put them through the finest screen of a food mill to remove the seeds. Combine the puree with sugar and lemon juice. Bring to a simmer and cook for 5 minutes.
Finely chop the chocolate and stir it into the raspberries off the heat. Scrape the jam into a bowl, cover and refrigerate overnight.
Assemble dishwasher-clean ½ pint jars and lids with new heat-sealing gaskets.
Reheat the jam to a boil in a heavy saucepan. Cook for 3 minutes at a simmer, stirring frequently. Off the heat, pour into jars. Attach the lids, screw on caps and invert the jars for 15 seconds. Return, right-side-up to a rack to cool.