"This was my great-grandmother's recipe handed down from generation to generation. This was always served on special occasions. It was one thing us kids looked forward to when visiting grandma's house."
Mix sugar, flour, and cocoa powder together in a saucepan. Stir in 1/2 cup milk slowly. Add egg. Stir in remaining milk. Cook over medium heat until thick, 3 to 5 minutes. Remove from heat; add butter and vanilla extract.