Heat the milk on the stove or in the microwave until almost boiling, you want it VERY hot
In a medium saucepan combine the flour, sugar, salt and the coconut and mix.
Once mixed, slowly begin adding the ALMOST BOILING 1 cup of milk into the mixture while whisking constantly as you pour.
Keep on a low heat, then as you see the filling start to thicken, begin adding the second cup of the HOT milk continuing to stir constantly until it starts to re-thicken.
Repeat this for your final cup of HOT milk, remember to stir it CONSTANTLY until it thickens for the third time.
Remove from the heat
In a medium bowl have your 3 eggs that are slightly beaten and pour your coconut mixture onto the beaten egg yolks, while STILL whisking constantly.
Cook together for about 2 minutes or until it starts looking like a 'pudding' then remove from heat, it should be hot, almost boiling, but careful not to burn it!
Stir in the 4 tablespoons of butter and the vanilla extract.
Allow to cool on the counter at least 30 minutes before pouring into your pie crust
(if you chose to use a pre-made DRY pie crust, you do not need to wait to pour, if you used a pillsbury one, wait the 30 minutes)
Chill the pie completely in the fridge for at least 3 hours before adding the whipped topping