In a saucepan over medium heat combine cream, butter, brown sugar, syrup and salt. Whisk and slowly bring to just a simmer then add chocolate and cocoa (you don't want to heat this fast and bring to a rapid boil using high heat - using medium heat, once you start to see the first few bubbles then add the chocolate and cocoa).
Reduce heat to low and stir until mixture is smooth and chocolate is melted, about 3 minutes.
Remove from heat, stir in vanilla and let cool for 10 minutes before serving.
To store, cool completely and refrigerate for up to 2 weeks. Reheat before serving.
For a sweeter version, substitute in a bit of milk chocolate. Or you can make it less sweet by substituting in some or all 70% dark chocolate. Makes 2 cups