Combine cookie crumbs and butter, mixing well. Press firmly into bottom and sides of a lightly greased 10-inch pie plate. Bake at 300° for 10 minutes. Let cool completely.
Combine sugar, cornstarch, and salt in top of a double boiler. Beat egg yolks; add to sugar mixture, stirring well. Gradually add scalded milk. Bring water in bottom of double boiler to a boil. Reduce heat to low; cook, stirring constantly, until mixture is slightly thickened and coats a metal spoon. Remove from heat.
Remove 1 1/4 cups hot mixture; place in a small mixing bowl. Add chocolate squares; stir until chocolate melts. Cool slightly. Add vanilla; stir well. Pour filling into prepared piecrust. Chill until slightly set.
Dissolve gelatin in cold water; add to remaining hot mixture, stirring constantly. Let mixture cool. Stir in rum.
Beat egg whites (at room temperature) until foamy; add cream of tartar. Beat until stiff peaks form. Fold into cooled gelatin mixture. Spoon over chocolate mixture; chill until set. Sprinkle with grated chocolate, if desired. Pipe whipped cream around edge of pie with a pastry bag.