in a large saucepan off heat, whisk the sugar, cornstarch, cocoa and salt. Slowly add 1 cup milk, whisking to make a paste-like consistency. Gradually whisk in the remaining milk until smooth.
Whisk the egg and vanilla into the milk mixture. Cook over medium heat, stirring, until gravy thickens and comes to a boil, about 15 minutes. Remove from the heat; whisk in the butter. To serve, pour warm chocolate gravy over halved biscuits.
The Do's and Don'ts of Cooking with Chocolate