In a large saucepan, bring whipping cream, corn syrup, dark brown sugar, salt, and HALF of the finely chopped chocolate to a boil over medium heat. Stir often until chocolate has melted. Reduce the heat under the pan and simmer (a low boil) for 5 minutes, making sure to stir the mixture frequently to prevent scorching.

Remove pan from the heat. Add butter, vanilla extract and the rest of the chocolate. Stir until chocolate is smooth and lump-free. Let the hot fudge sauce cool just a bit before serving.

Store the unused sauce in a covered jar or container with a lid for up to a week in the refrigerator. Once refrigerated, the sauce will thicken considerably. When ready to use it again, place the amount you will be using in the microwave (in a microwave-safe container) and re-heat for 10-15 seconds. The sauce can also be reheated on your stovetop. Enjoy!