Prepare boiling water canner. Heat jars in simmering water until ready to use, do not boil. Wash lids in warm soapy water and set aside with bands.

Combine raspberries and lemon juice in a large stainless-steel saucepan. In a small bowl whisk cocoa powder and pectin together; add to raspberries stirring to combine. Bring mixture to a rapid boil over high heat, stirring frequently. Add the sugar and return to a full rolling boil that cannot be stirred down, boil hard for 1 minute. Remove from heat. Skim foam if necessary.

Ladle hot jam into a hot jar, leaving a 1/4 inch headspace. Remove air bubbles. Wipe the jar rim. Center lid on the jar, apply band and adjust to fingertip-tight. Place the jar in the boiling-water canner. Repeat until all jars are filled.

Water must cover jars by 1 inch. Adjust heat to medium-high, cover canner, and bring water to a rolling boil. Process half-pint jars 10 minutes, adjusting for altitude. Turn off heat and remove cover. Let jars cool 5 minutes. Remove jars from canner; do not retighten bands if loose. Cool 12-24 hours. Check lids for seal, they should not flex when center is pressed.