Add chocolate to milk and heat in double boiler. When chocolate is melted, beat with rotary egg beater until blended. Combine sugar, flour and salt; add gradually to chocolate mixture and cook until thickened, stirring constantly; then continue cooking 10 minutes, stirring occasionally.
Pour small amount of mixture over egg yolks, stirring vigorously; return to double boiler and cook 2 minutes longer. Add butter and vanilla and cool
Turn into pie shell; chill. Pile whipped cream lightly around edge of pie to form a 1-inch border. Sprinkle cream with grated orange rind.