
Whisk together the sugar, flour, and cocoa powder in a medium, heavy-bottomed saucepan. Whisk in the whole milk until almost smooth.
Place the pot over medium heat and cook, whisking occasionally, for 5 to 7 minutes or until the mixture thickens and bubbles.
Once this mixture is bubbling, continue to cook for 1 more minute, whisking constantly.
Remove the pan from the heat and stir in the butter and vanilla extract, until the butter is melted.
Serve over biscuits.