Combine the heavy cream, butter, brown sugar, corn syrup and salt in a saucepan. Bring to a simmer over medium heat, whisking. Add the chocolate and cocoa powder, reduce the heat to low and cook, whisking, until the chocolate is melted and the sauce is smooth, 1 to 2 minutes.
Whisk in the vanilla. Let cool 10 minutes before serving. To store, let cool completely, then cover and refrigerate up to 2 weeks; reheat before serving.