Combine cream, syrup, brown sugar, cocoa powder, salt and half of the chopped chocolate in a medium saucepan. Bring to a boil on medium heat; reduce the heat to medium-low and cook for 5 minutes, stirring occasionally.
Remove the pan from heat and stir in the remaining chopped chocolate, butter and vanilla extract. Stir until smooth. Cool for 20~30 minutes before using it. The sauce will thicken as it cools. Store in an airtight container in the fridge for up to a month. Reheat in the microwave to get it back to pourable consistency.