In a medium saucepan, stir the heavy cream, corn syrup, brown sugar, cocoa powder and salt. Bring the mixture to a boil and stir in half the chopped chocolate. Turn the heat down and simmer for 5 minutes. Turn the heat off and stir in the remaining chocolate, butter and vanilla. Allow to cool fro 15-20 minutes before using.

Store in an airtight container for up to 2 weeks. To reheat, microwave at 15-30 second intervals, stirring each time, until melted.