Step 1

In a medium saucepan, combine sugar, flour and salt over a medium heat; gradually stir in milk. Cook and stir over medium heat until the mixture is thick and bubbly. Reduce heat to low and cook 2 minutes more. Remove the pan from heat.

Step 2

Place a strainer over a clean mixing bowl; set aside.

Step 3

Beat the egg yolks slightly. Gradually pour 1 cup of the hot custard mixture into yolks, whisking constantly. Return the egg mixture to the saucepan and bring the entire mixture to a gentle boil. Cook and stir 2 minutes before removing the pan from heat. Immediately pour custard through the strainer.

Step 4

Stir butter, vanilla, and coconut into the hot mixture. Pour the hot filling into the baked pie crust. Cool and refrigerate until set, about 4 hours.