1. Combine chocolate chips, butter, condensed milk, and salt in the top of a double boiler over medium heat on stovetop, stirring constantly until chocolate chips are melted and the mixture thickens, approximately 10 minutes.

2. Remove from heat. Stir in water until sauce reaches desired consistency. Cool slightly. Stir in vanilla.

3. To store sauce, cover and place in refrigerator. To reheat, stir in a small amount of water and microwave at 30% power, stirring after each minute (heating time will depend on amount of sauce).