In a medium, heavy saucepan over medium heat, combine cream, corn syrup, brown sugar, cocoa, salt, and half of the chocolate. Bring the mixture to a boil, stirring until the chocolate pieces are completely melted.
Reduce heat and cook at a low boil, stirring occasionally, for about five minutes. Remove the pan from the heat and add in butter, vanilla, and remaining chocolate, stirring until smooth.
Allow the sauce to cool down before serving.
Notes
Makes about 2 cups.
This recipe can be easily adapted to create new flavors. I added in 1/4 teaspoon of peppermint extract which yielded a subtle, smooth, minty flavor.
To store, cool sauce completely and chill in an airtight container within the refrigerator for up to one week.