In a medium saucepan, combine the sugar, cocoa, and flour with a whisk. Place over medium-low heat, and add the milk in a steady stream while whisking constantly. Bring to a simmer and cook, stirring constantly, until the mixture reaches the consistency of a thin pudding, about 5 minutes. The gravy should coat the back of your spoon and leave a slight line when you drag the spoon through the middle of the pot. Remove from heat and stir in the butter, vanilla, and salt until combined. Serve hot over biscuits. You may keep warm over very low heat on the stovetop, stirring occasionally and adding a splash of milk if the gravy gets too thick.