In saucepan mix sugar, flour and salt.
Add half of milk; mix smooth. Bring to boil over medium heat, stirring briskly. Continue to stir and boil until smoothly thickened, about 2 minutes. Remove from heat.
With fork, beat eggs with remaining milk; gradually stir into hot mixture, then put back over heat.
Bring to a boil, stirring, and boil until mixture thickens a bit more, about 1 minute. Remove from heat, stir in butter, chocolate and vanilla. Stir until chocolate is completely melted.
Pour into baked shell; chill.
Spread with sweetened whipped cream; garnish with shaved chocolate.