Mix the cocoa, sugar, and flour together. Add the milk, making sure it mixes smoothly. Heat (medium) and stir constantly until it thickens. This takes about 8 minutes. Take off of heat. Add butter. Stir until it is melted and mixed. Add vanilla and stir until mixed. Serve hot over buttered biscuits. Buttered toast will do in a pinch. Notes: This is an almost lost Ozark classic. It hardly ever shows up in ethnic or regional cookbooks. If this cools, it becomes something else: chocolate pudding! Chocolate gravy is a great way to make sure your kids get their minimum daily requirement of sugar!