Place a saucepan on the stovetop. Pour 2 cups of the milk into the saucepan, reserving the other 1/2 cup. Sprinkle 1/2 cup sugar evenly over the milk to cover the entire surface area of the bottom of the pan. Add a dash of salt. Do not stir. Turn the burner on to medium. Leave the milk and sugar undisturbed until it begins to bubble and boil, while you do the next step.
Meanwhile, in a small bowl, whisk together the remaining 1/2 cup milk, egg yolks, and cornstarch until smooth and yellow. As soon as the contents of the pan begin to boil, pour the egg yolk and cornstarch mixture directly into the pan and being whisking immediately. Whisk continuously for 60 seconds, until the pudding thickens. Remove the pudding from the heat and whisk in the vanilla and butter until melted and smooth. Pour the pudding into 6 ramekins. Refrigerate to chill through.
Note: The pudding can also be used as the base for a cream pie, icebox cake, or trifle. At our house, we enjoy it with banana slices and crumbled graham crackers or vanilla wafers.