In a medium bowl, sift or whisk together sugar, cocoa powder, and flour. Set aside.
In a heavy skillet, melt butter over medium heat. Whisk in the dry ingredients, cooking and stirring for about 1 minute.
Gradually whisk in the milk and vanilla extract. Simmer, whisking constantly, until the gravy thickens, about 4-5 minutes.
Serve over hot biscuits.
Notes
This recipe yields about 3 cups of gravy.
Bake your biscuits before preparing the gravy. The gravy comes together quickly, and is best when served warm -- straight from the skillet.
Use whole milk for the best flavor and texture. If necessary, 2% milk will also work. I don't recommend skim milk.
Gradually add the liquid to the skillet in small amounts, whisking constantly. This gives the flour mixture time to absorb the liquid slowly, preventing a lumpy gravy.
Use a cast iron skillet (or other heavy pan), and whisk constantly. This helps to avoid scorching in the bottom of the skillet.