In a medium saucepan, whisk together the cocoa powder, flour, sugar, and salt. Pour in the milk and whisk until all the lumps are gone. Place the pan over medium heat and cook 5 to 7 minutes, stirring constantly, until the gravy just begins to boil and then thickens. Once thick, remove from the heat stir in the butter and vanilla. Stir until the butter has melted. Serve over hot biscuits, pound cake, or even over ice cream!