In 2-quart stainless steel or nonstick saucepan (do not use aluminum because pudding will discolor), stir together cornstarch and sugar. Gradually stir in soymilk and egg yolks with wire whisk. Heat to boiling over medium heat, stirring constantly. Boil 1 minute, stirring constantly; remove from heat. Stir in vanilla and butter. Cover surface of pudding with plastic wrap to prevent skin from forming. Refrigerate about 30 minutes or until cool.