In a 1 1/2 quart saucepan combine cream, corn syrup, brown sugar, cocoa powder, salt and 3 oz. of the chocolate. Bring mixture to a light boil over medium heat, reduce heat and let simmer, stirring frequently, about 5 minutes. Remove from heat, add in remaining 2 oz. chocolate, butter and vanilla extract and stir until smooth. Let cool to warm. Serve over ice cream. Sauce can be store in an airtight container in refrigerator for 1 week. Reheat before serving.
