Combine, sugar, cornstarch and salt with a whisk...add milk. Stir until smooth...put in a microwavable deep dish (I use an 8 cup glass measuring bowl). Microwave on hi for 7 minutes...remove and stir really good with the whisk...mixture should be thick. Beat egg yolks and temper the custard mixture with the yolks..mix all together and microwave on hi for 1 more minute.
Remove, add 1 cup coconut, butter and vanilla...stir well, pour into prepared pie shell.