In 2-quart saucepan, mix 2/3 cup sugar, the cornstarch and salt. In large bowl, beat milk and egg yolks with wire whisk until blended; gradually stir into sugar mixture. Cook over medium-low heat about 15 minutes, stirring constantly, until mixture thickens and boils. Boil 2 minutes, beating constantly with wire whisk; remove from heat. Beat in 2 tablespoons butter and 1 tablespoon vanilla with wire whisk. Press plastic wrap on filling to prevent a tough layer from forming. Cool at room temperature while baking crust.