Using a double boiler, bring the water in the bottom pot to a boil.

Place the chocolate squares and butter in the top of the double boiler.

When they are melted, begin to stir in the sugar.

Add ½ cup at a time, mixing each one until it is well combined. (This is important to make sure the final sauce is well homogenized.)

When all the sugar is incorporated, it will look like dark brown beach sand.

Once all the sugar is in, begin to add the condensed milk a little at a time.

Again, stir until it is completely mixed in.

As you add the milk, the sauce will become smooth and velvety. Keep cooking it until this texture is achieved or it will be grainy.

Then add the vanilla.

Store in jars with lids in the refrigerator.

To serve, heat the sauce. Whole jars can be immersed in hot water until the sauce is warm. Stir several times to speed the warming of the entire jar.

Small amounts for single servings can be heated in a microwave.