Combine sugar, flour, salt and 1/2 cup cold milk; mix well. Scald remaining 1 1/2 cups milk and add chocolate. Stir until chocolate is melted. Add scalded milk and chocolate to the first mixture and cook until thickened.
Beat egg yolks slightly and add slowly to hot mixture, stirring constantly. Continue to cook over low heat for 2 to 3 minutes. Remove from heat and add remaining ingredients except pie shell.
Cool and pour into baked pie shell. Chill and top with whipped cream.