Transfer condensed milk to three 1/2 pint mason jars and put on the lids and rings.

Place in slow cooker and cover with water. (About an inch above the jars)

Cook on low for about 8 hours.

Remove from water and allow to cool until the jars are room temperature.

Carefully remove rings and lids.

Add 1/4 tsp of vanilla to each jar (optional).

Stir well until sauce is smooth.

To store, replace, lid and ring and store in refrigerator for up to a month.