How to Make It
Sift the cocoa, sugar, flour, and salt into a large skillet, preferably a cast iron skillet. Add the milk in a slow, steady stream, whisking until smooth.
Cook over medium heat, stirring continuously with a heatproof spatula or wooden spoon, until the gravy thickens to the consistency of thin pudding, about 8 minutes. The gravy will thicken around the edge first, so make sure to keep it stirred up from the bottom and sides.
Remove the skillet from the heat and drop in the butter. Stir until melted and smooth. Serve the warm gravy with the biscuits.