In 2-quart saucepan, combine sugar, flour, and salt. Stir in milk. Cook over medium heat, stirring until mixture thickens and boils, about 10 minutes. Boil 1 minute.
In small bowl with fork, beat egg yolks slightly. Beat small amount of milk mixture into yolks. While stirring, slowly pour egg mixture back into milk mixture. Cook over medium-low heat, stirring until mixture thickens and coats spoon well, about 8 minutes (do not boil).
To check thickness, lift metal spoon from mixture and hold up for 15 seconds. Spoon should not show through mixture. Remove from heat and stir in vanilla or almond extract. Cover surface with plastic wrap; chill well, about 2 hours.
In small bowl, with mixer at medium speed, beat heavy cream until stiff peaks form. With rubber spatula, gently fold cream into custard. Makes 4 2/3 cups.