Pour the sweetened condensed milk into the Mason jars, filling to the top. Tightly screw lids on.
Place upright in a large crock pot. Pour hot tap water into the crock pot to cover the jars. There should be about 1 inch of water above the lid of the jars.
Place lid on crock pot and cook on LOW for 8 to 10 hours, depending on how dark and thick you want it.
Carefully remove jars from crock pot using tongs. Place on counter and let cool to room temperature. A little rust may form on the lids. If so, wipe it off with a paper towel.
Store in the refrigerator. It will keep for at least 2 weeks. Reheat in the microwave to make it pourable.