Toast 1/2 cup coconut in a dry skillet over medium-low heat for 4 minutes until golden, then cool it.
Whisk the granulated sugar, cornstarch, salt, milk and egg yolks in a saucepan and cook over medium heat for 8 to 10 minutes until thick and 170°F.
Remove the custard from the heat, stir in the butter, vanilla, toasted coconut and 1/4 cup untoasted coconut and spread it in the crust.
Press plastic wrap directly against the filling and refrigerate for at least 4 hours until cold and firm.
Whip the cream with the powdered sugar to medium peaks, spread it over the pie and finish with the remaining untoasted coconut.