First, combine sugar, flour, milk and chocolate shavings in a 2 quart sauce pan. Stirring constantly, cook over medium heat until mixture bubbles. Stir for 2 minutes.

Mix a little of the hot mixture into the whisked egg yolks. Beating rapidly to avoid cooking the yolks. Stir the warm yolk mixture back into the remainder of the chocolate mixture. Cook for an additional 90 seconds. Remove from the heat, add in your butter chips and your vanilla.

Pour filling into the pie shell. Let the pie set for 1 hour outside of the refrigerator then 2 hours in the refrigerator.