For the pie filling:

1. Blend sugar, cornstarch, flour, and salt.
2. Stir gradually into hot milk, and cook over boiling water (using a double boiler) until thickened.
3. Add chocolate, and stir until well blended and smooth.
4. Add a small amount of cooked mixture to beaten egg yolks, blending in thoroughly. Return this mixture to the remaining mixture in a saucepan and cook for 2 minutes longer.
5. Remove from heat then add the vanilla, and mix well.
6. Place cooled filling into the pie shell, then make the meringue. 
7. Pile the meringue on top of the filling, going to the edge of the pie with the meringue to prevent shrinking and gaps.
8. Bake at 375 degrees F for 12-15 minutes or until the meringue is lightly browned.

For the meringue:

1. Beat egg whites until stiff and foamy.
2. Add sugar very slowly (this is important) to preserve the air in the meringue, continuing to beat until soft peaks are formed. Add the vanilla while beating to mix well.
3. Spread the meringue over the filling. Seal to the edge of the crust to prevent shrinking.