In a small saucepan combine the cream, dark brown sugar, cocoa powder, corn syrup and salt.

Whisk over medium-high heat, being sure to scrape the bottom and sides of the saucepan, until the mixture begins to boil; cook for 30 seconds, then remove from the heat.

Add the chocolate and butter and whisk until melted and glossy.

Add the vanilla and whisk to combine.

Serve warm; refrigerate up to one month, warming in a microwave or water bath between uses.