Combine sugar, cornstarch and salt in medium saucepan. Gradually stir in evaporated milk to dissolve cornstarch. Whisk in egg and yolk. Add butter. Cook over medium heat, stirring constantly, until the mixture begins to thicken. Reduce heat to low; bring to a simmer and cook for 1 minute, stirring constantly. Remove from heat. Stir in vanilla extract. Let cool slightly.