In a 2-quart saucepan over medium-high heat, bring the cream, syrup, brown sugar, cocoa powder, salt and half of the chocolate to a boil. Reduce the heat to medium-low or low (enough to maintain a low simmer), and cook for 5 minutes, stirring occasionally.
Remove from the heat and stir in the remaining chocolate, the butter, and the vanilla extract, stirring until smooth. Let cool for 20 to 30 minutes before using (it will thicken as it cools). Store in a jar or airtight container in the refrigerator for up to 2 weeks. To reheat, microwave for 30 seconds to 1 minute until it's pourable but still thick.