Heat oven to 425~. Reserve 1/2 oz of chocolate for cake decoration. In heavy saucepan on low heat, melt broken chocolate with butter, stirring constantly; set aside. In top of double boiler, beat eggs with sugar until mixture is lukewarm and sugar is dissolved. (Do not overcook.) Remove from heat and beat until quadrupled and very thick. Blend flour into egg mixture; fold into chocolate mixture, a little at a time. Spread into buttered and waxed paper lined 8" cake pan. Bake at 425~ for no longer than 15 minutes.
(Center of cake will be soft.) Cool. Freeze cake overnight before removing from pan. Unmold. Cover with sweetened whipped cream, if desired. Decorate with shaved chocolate. Refrigerate until serving time.