Prep the chocolate and the butter - break the chocolate into bite-sized pieces. Slice the butter into pats or little cubes. Add the chocolate and butter to a small bowl, and microwave for only 20 seconds. It will be just enough. Set aside.
Mix the ingredients - In a medium saucepan on low heat, combine the sugar, cocoa powder, cinnamon, cayenne, salt, and cornstarch. Mix well. Add in the milk, and whisk until combined.
Bring to a boil - Increase heat to medium, stirring constantly until the mixture reaches a boil. Remove from the heat immediately.
Finish - Stir in the chocolate pieces, butter, and vanilla. Allow mixture to cool a bit, stirring occasionally to prevent a skin from forming. Serve warm.