Line the bottom and sides of a waxed paper-lined and greased 9-inch loaf pan with ladyfingers, reserving remaining ladyfingers. Set aside.
Place chocolate squares in top of a double boiler. Place over boiling water; cook, stirring frequently, until chocolate melts. Add sugar, water, and egg yolks; cook, stirring constantly, until mixture is smooth and thickened. Remove from heat, and stir in mint extract. Cool mixture to room temperature.
Beat egg whites (at room temperature) until stiff peaks form. Fold egg whites and 1 cup whipped cream into chocolate mixture; spoon into prepared pan. Top with remaining ladyfingers. Cover; chill overnight.
Unmold cake onto a chilled serving platter; frost with additional whipped cream, if desired. Slice and serve immediately.