In a saucepan, combine the sugar, flour, and salt. Add milk in gradually while stirring gently. Cook over medium heat, stirring constantly, until the
mixture is bubbly. Keep stirring and cook for about 2 more minutes, and then remove from the burner.
Stir a small quantity of the hot
mixture into the
beaten egg yolks, and immediately add egg yolk
mixture to the rest of the hot
mixture. Cook for 2 more minutes; remember to keep stirring. Remove the
mixture from the stove, and add butter and vanilla.
Stir until the whole thing has a smooth consistency.