In saucepan, combine sugar, flour and salt: gradually stir in milk. Cook and stir over medium heat until mixture boils and thickens. Cook 2 minutes longer. Remove from heat. Stir small amount of hot mixture into yolks.  Return to hot mixture and cook 2 minutes stirring constantly. Remove from heat. Add butter, coconut and vanilla. Cool to room temperature. (To prevent a crust from forming, put clear plastic wrap or waxed paper directly on top, touching surface of the hot pudding clear to sides of bowl.) Pour into a baked pastry shell.