Fill a waterbath canner with water, and place inside eight half-pint jars (make sure the water covers the jars). Bring to a boil. Place lids and rings in a small saucepan with hot water and heat, but do not boil.

Bring cantaloupe, lemon juice, and 3 1/2 cups of sugar to a boil in a large stock pot over medium-high heat. Bring to a full, rolling boil that cannot be stirred down (it should take 10-15 minutes).

Once the mixture is at a full boil, whisk together the remaining sugar and the pectin. Whisk the mixture into the cantaloupe mixture.

Bring mixture back to a full boil, and then boil hard for 2-3 minutes, or until the mixture looks thickened and is set. I like to test it by putting a small amount on a spoon and placing it in the freezer for a few minutes. If it's jelly-like when it's cold, it's set! If not, boil for a few more minutes.

Once set, remove from heat, and stir in the vanilla extract and salt.

Remove the hot jars from the waterbath canner, and turn the canner back up onto high.

Ladle the jam into the hot jars, leaving a 1/2" headspace. Using a clean, damp cloth, wipe any extra jam from the rim of the jars, and then place on the lids and the rings—tightening just until snug, not overly tight.

Place the jars in a rack in the waterbath canner, bring to a boil, and process for 10 minutes. Remove from canner, and let cool completely. Check seals after 24 hours—the lids shouldn't flex or move when pushed down on. Store sealed jars in a cool dark place for up to a year. Any jars that don't seal, place in the fridge and eat within a month.