Combine sugar, cornstarch, and salt in a heavy saucepan; stir in milk. Cook over medium heat, stirring constantly, until thickened and smooth.
Beat egg yolks until thick and lemon colored. Gradually stir about one-fourth of hot milk mixture into yolks; add to remaining hot milk mixture, stirring constantly. Cook 5 minutes, stirring constantly. Remove from heat; add butter and vanilla. Cool. Stir 2/3 cup coconut into cooled filling. Pour filling into pastry shell; chill overnight.
Beat whipping cream until foamy; gradually add powdered sugar, beating until soft peaks form. Spread over pie: sprinkle with remaining coconut.