This pie has a rich layer of caramelized milk, which you can make in the oven or in a slow cooker. To use a slow cooker, take the paper wrapper off the can of condensed milk and place it in your slow cooker. Cover the can by at least 2 inches of water. Turn to low and cook for 5 hours. Cook more than one at a time, so when the mood strikes, you can just pull a can of cooked milk out of the refrigerator; it will keep for 1 month refrigerated.