Mix sugar, corn starch and salt together in a small saucepan. In a separate bowl combine milk and egg yolks. Slowly pour milk and egg into sugar mixture, stirring as you pour. Cook over medium heat until the mixture thickens and begins to boil, stirring constantly. Continue stirring and boil for 1 minute. Remove from heat and add butter and vanilla. Pour into 4 small serving bowls, cool at room temperature and then chill for at least 2 hours before serving.