In a large saucepan, combine milk and butter and bring to a boil over medium heat.
Remove from heat.
In a large bowl, combine eggs, sugar and cornstarch. Beat on medium speed with mixer for 5 minutes.
Add hot milk to mixture, in thirds, whisking well to prevent curdling.
Return mixture to saucepan.
Reduce heat to medium-low and cook, stirring occasionally until mixture thickens, approx. 1o minutes.
Remove from heat, and add vanilla, stirring to combine.
Fill a large bowl halfway with ice and water.
Transfer hot custard to a stainless steel bowl and set bowl in ice water to cool to room temperature. Cover custard and let refrigerate for 1 to 2 hours.
At this point you can make your pie crust per recipe or package directions.
Allow to cool.
When custard mixture has cooled and thickened, add coconut, stirring to combine.
Spoon custard into cooled crust. Top with whipped topping and sprinkle with toasted coconut.