Put 1/2 cup of milk in a mixing bowl and sprinkle in the gelatin. Allow it to soften.
Stir egg, egg yolks, cornstarch, and sugar into the milk mixture.
Put remaining 2 1/2 cups of milk in a non-reactive saucepan over high heat to scald the milk. When milk is scalded, gradually whisk it into the gelatin mixture.
Pour the mixture back into the saucepan and return to the stove, whisking all the time. When the mixture begins to boil and thicken, remove from heat and add butter, vanilla, salt, and coconut.
Whisk until all the butter is melted and incorporated. Place custard in a container to cool. Cover with plastic wrap against the custard to keep a skin from forming. Alternatively, place custard in an 8 inch square pan and put it in a larger pan with ice water coming up the sides. This will help it cool quicker.
When the custard is cool, whip 1 cup of cream until peaks start to form, then fold into the chilled custard until smooth. Try not to overmix. Pour mixture into prebaked crust and chill for several hours or until very cold. Sprinkle top with coconut.
If desired (and I always do this), whip another 1/2 to 1 cup of cream, sweetened to taste with powdered sugar, add a little vanilla and spread over the top of the pie. If you like frozen whipped topping you can thaw out some Cool Whip and use that instead. Sprinkle more toasted coconut over the pie.