In a medium saucepan, stir together sugar, cocoa, flour, and cinnamon and salt. With a wire whisk or spoon stir in milk a little at a time until ingredients are wet. Cook over medium heat, stirring constantly, until mixture thickens. (This will burn easily so be sure to stir). Remove from heat and stir in butter and vanilla until smooth. Pour into a bowl and top with nuts if desired. Serve over warm biscuits.