PREPARE boiling water canner. Heat jars and lids in simmering water until ready for use. Do not boil. Set bands aside.
COMBINE cocoa powder and pectin in a medium glass bowl, stirring until evenly blended. Set aside.
COMBINE crushed raspberries and lemon juice in a large stainless steel saucepan. Whisk in pectin mixture until dissolved. Bring to a boil over high heat, stirring frequently. Add sugar all at once and return to a full rolling boil, stirring constantly. Boil hard for 1 minute, stirring constantly. Remove from heat and skim off foam.
LADLE hot sundae topper into hot jars leaving 1/4 inch headspace. Remove air bubbles. Wipe rim. Center lid on jar. Apply band until fit is fingertip tight.
PROCESS jars in a boiling water canner for 10 minutes, adjusting for altitude. Remove jars and cool. Check lids for seal after 24 hours. Lid should not flex up and down when center is pressed.