Fill crockpot with COLD water. Put an UNOPENED can (with paper label removed) of sweetened condensed milk (not evaporated milk) into the cold water, making sure that the water is 2" taller than the can. If you are doing more than one can, make sure there is at least 2" of space BETWEEN each can as well.
Put the lid on the crockpot and let it "cook" on LOW for EIGHT HOURS. Take the cans out of the crockpot and dry them off and label them. If you are going to use them right away, let them cool down a couple hours before you try to open the can. Cooled and unopened cans can be kept in the pantry like you would any other canned item.
Crockpot Dulce de Leche
Right after opening the can; it is unbelievably delicious!!
NOTE: One friend that had a white crockpot, said that the can left a permanent "ring" on the bottom of her crockpot; I didn't have that problem (my crockpot is black). If you are concerned, put a layer of paper towels under the can/cans before you add the cold water.