In a one quart saucepan, stir together cocoa powder, brown sugar, syrup, cream, salt and half of the chocolate.
Set the pan over medium heat and cook, stirring constantly, until chocolate melts and mixture begins to boil (bubbles will appear around side). Allow mixture to come to a low boil and let simmer for about 3 minutes, stirring often. .
Remove from heat and stir in butter, remaining chocolate and vanilla.
Pour into a jar and store in the refrigerator.
Notes
I usually use light brown sugar, but dark brown sugar is the original.