Beat egg whites until soft peaks, set aside. In a saucepan stir together sugar, flour and salt. While stirring gradually add milk. Cook over medium heat stirring constantly until it bubbles around the edge of pan. Cook and stir 2 minutes. Remove from heat. In a small bowl slightly beat the egg yolks. Temper eggs by adding a small amount ( a couple of tablespoons) of hot mixture into yolks. (I do this about 3 times) Return to pan and cook 2 minutes. Add vanilla, butter and egg whites. Cook and stir 1 minute.
Pour into cooled baked pastry shell. Place plastic wrap over pudding just touching the mixture. This keeps it from forming that thick dry layer on top. Cool.
Just a fun note. My grandmother baked my grandfather a pie every day for the first year they were married. Crazy huh?