Mix 1/2 cup sugar, the cornstarch and salt in 3-quart saucepan. Gradually stir in milk and sherry. Heat to boiling over medium heat, stirring constantly. Boil and stir 1 minute. Gradually stir at least half of the hot mixture into egg yolks, then stir back into hot mixture in saucepan. Boil and stir 1 minute; remove from heat. Stir in butter and vanilla. Cover and refrigerate at least 3 hours, but no longer than 24 hours.