In a saucepan, combine sugar, flour and salt. Stir in milk until smooth; cook and stir over medium high heat until thickened and bubbly. Cook and stir 2 minutes longer.
Remove from the heat; gradually stir about 1 cup of hot mixture into beaten eggs. Return all to saucepan. Bring to a gentle boil. Cook and stir 2 minutes more.
Remove from the heat; stir in 1 cup coconut, butter and vanilla. Pour into pastry shell; sprinkle with remaining coconut.
Chill for several hours before serving. Refrigerate leftovers.