Combine flour, sugar, salt, and milk in large saucepan, cook over medium heat, stirring constantly until thickened.
Beat egg yolks in small bowl. Gradually stir a few spoonful's of hot mixture into egg yolks to warm, then add yolks back into hot mixture. Cook until very thick, stirring constantly.
Remove from heat, stir in margarine & rum extract.
Divide custard in half to second layer.
Spread pie with topping, and decorate with shavings of chocolate.
Refrigerate until serving time.
Topping: 1 cup whipping cream, whipped. Add 2 teaspoons sugar & 2 teaspoons rum extract to whipped cream.