In a medium saucepan, combine granulated sugar, cornstarch, and salt. Gradually whisk in the whole milk until smooth. Cook over medium heat while stirring for about 5 to 7 minutes until thickened.

In a separate bowl, whisk the egg yolks. Slowly add ½ cup of the hot custard into the yolks, whisking continuously, then pour the yolk mixture back into the saucepan.

Stir in unsalted butter, vanilla extract, and sweetened shredded coconut. Cook for an additional two minutes.

In a clean bowl, beat the egg whites with cream of tartar on high speed until soft peaks form.

Gradually add in granulated sugar, continuing to beat until stiff peaks form, then add vanilla extract.

Spoon the meringue over the warm coconut filling, sealing it to the edges of the pie crust.

Bake in a preheated oven at 350°F (175°C) for 12-15 minutes until golden brown.

Allow the pie to cool for one hour at room temperature, then refrigerate for at least 3 hours before serving.