Combine sugar, flour, and salt in top pan of double boiler.
Gradually stir milk into above mixture and set over hot water.
Stir with whisk until thickened thoroughly (about 5-10 minutes).
Cover, and cook 10 minutes, stirring occasionally with whisk to keep smooth.
Stir a small amount of hot mixture into slightly beaten egg yolks.
Stir into mixture in double boiler.
Cook about 2 minutes, stirring continuously.
Add butter and vanilla.
Once butter is melted, remove mixture from heat and cool slightly.
Pour into cooled, baked pastry shell.
Bake at 350℉ until lightly browned (approximately 15 minutes).
Chill for about an hour before spreading whipped topping over the top.
Chill for an additional 30 minutes before serving.
Top with shredded coconut before serving.
Notes
To scald milk, add milk to a shallow saucepan. Heat over medium heat and stir frequently. When small bubbles appear around the outside of the saucepan or when the temperature is 180-185 degrees Fahrenheit, the milk is scalded.
Use a pie crust shield to keep the edge of the crust from burning.
For best results, chill for at least an hour before serving.