In a mixing bowl, measure the sugar, cocoa powder, flour, cinnamon and salt. Whisk to combine.
Add the milk and melted butter to the cocoa mixture and whisk until smooth. Then pour the mixture into a small sauce pot.
Set the sauce pot over medium heat. Simmer the gravy to thicken, whisking periodically, for at least 5 minutes, but up to 10 minutes. This ensures the flour cooks through and does not taste raw. If the gravy gets too thick, add a splash of milk to thin it out a little.
When ready to serve, whisk in vanilla extract. While it's still hot, spoon the gravy over warm cinnamon biscuits and serve.