Combine first 3 ingredients in a 4-qt. stainless steel or enameled Dutch oven. Bring mixture to a full rolling boil that cannot be stirred down, over high heat, stirring constantly.
Meanwhile, stir together sugar and cocoa until blended; add all at once to boiling cherry mixture. Return mixture to a full rolling boil. Boil hard 1 minute, stirring constantly. Remove from heat. Skim foam, if necessary.
Ladle hot jam into a hot jar, leaving 1⁄4-inch headspace. Remove air bubbles. Wipe jar rim. Center lid on jar. Apply band, and adjust to fingertip-tight. Place jar in boiling-water canner. Repeat until all jars are filled.
Process jars 10 minutes, adjusting for altitude. Turn off heat; remove lid, and let jars stand 5 minutes. Remove jars and cool.