In a pan stir together brown sugar, cocoa powder, whipping cream, chocolate liqueur and half of the chocolate (84 g or 3 ounces), syrup and salt .
Cook over medium heat, stirring, until the chocolate has melted. Bring mixture to a boil, then reduce heat to low and allow to simmer for 5 minutes, stirring frequently.
Remove pan from heat then add remaining chocolate, butter and vanilla. Stir until smooth.
Leave to cool for 30 minutes before using. Store leftover sauce in an airtight container in the fridge for up to 2 weeks. To use refrigerated sauce, reheat in the microwave for 30 seconds to 1 minute.