Step 1: Prepare your canning jars and lids by washing them in hot, soapy water. Sterilize the jars by boiling them for 10 minutes, then keep them hot until ready to fill. Set lids aside in hot (not boiling) water.
Step 2: In a large, heavy-bottomed pot, combine the finely diced bell peppers, jalapeños, and apple cider vinegar. Bring to a boil over high heat, then reduce heat and simmer for 5-7 minutes, until peppers are tender.
Step 3: Add the granulated sugar to the pepper mixture, stirring constantly until the sugar is completely dissolved. If using, add the butter or margarine to help reduce foaming.
Step 4: Bring the mixture to a rolling boil over high heat, stirring constantly. A rolling boil is one that cannot be stirred down. Continue to boil for 1 minute.
Step 5: Immediately stir in the liquid pectin. Return to a rolling boil and boil for exactly 1 minute, stirring constantly. Remove from heat.
Step 6: Carefully skim off any foam from the surface of the pepper jelly using a metal spoon.
Step 7: Ladle the hot pepper jelly into the hot, sterilized jars, leaving 1/4 inch headspace. Wipe jar rims clean with a damp cloth, center lids on jars, and apply screw bands until fingertip tight.
Step 8: Process the filled jars in a boiling water canner for 10 minutes. Turn off heat, remove canner lid, and let jars stand for 5 minutes. Carefully remove jars and place them on a towel-lined surface to cool completely for 12-24 hours. Do not disturb them while cooling.
Step 9: Check seals. Lids should be concave and not flex when pressed. Store properly sealed jars in a cool, dark place for up to 1 year. Refrigerate any unsealed jars and consume within 3 weeks.