In a 2 quart saucepan, combine sugar, cornstarch, salt and mix to combine.
Add egg yolks and whisk until smooth and there are no visible lumps. Set aside.
Heat milk until warm to the touch. Do not boil. Remove from heat and slowly ladle the warm milk into the sugar/egg mixture while whisking constantly. Switch to a wooden spoon or spatula that will fit into the edges of the saucepan. Place saucepan over medium low heat and continue to cook, stirring constantly, until the mixture begins to thicken. Remove from heat and pour custard into a bowl. Stir in butter and vanilla.
To avoid a skin from forming, place a lightly buttered piece of plastic wrap directly on the top of the custard. Cool to room temperature and refrigerate for up to two days.
