Whisk in the flour and continue whisking until it browns just a little.
Add sugar and salt. Stir it quickly so it doesn't burn or scorch.
Add cocoa powder and stir.
Add the boiling water and whisk continually until smooth.
Add the milk and whisk again.
Let the sauce get hot, but don't let it boil (unlike pudding). Keep whisking until it just starts to thicken. When you feel it getting thicker, or see just a few bubbles, remove it from the heat.
Add the vanilla and stir to combine.
Let it cool just a bit (sauce will continue to thicken as it cools).
Pour the chocolate gravy over the top of the biscuit, pancakes, waffles, or eat by the spoonful. Enjoy!
Chocolate gravy can be stored in the refrigerator for two or three days. As I said, it really does taste best when eaten fresh. But making a large batch ahead of time is an option.
Reheat chocolate gravy in a pot on the stove over medium-low heat. Continually stir and whisk (once it's thin enough) to keep from scortching the gravy. You may need to thin it with some additional milk, as the gravy tends to thicken when cooled.