Combine gelatin and 1 cup sugar in top of a double boiler; mix well. Stir in egg yolks, lemon juice, and salt. Cook over boiling water, stirring constantly, 10 minutes or until thickened. Add butter, lemon rind, and vanilla, stirring until butter melts. Chill mixture until partially thickened.
Arrange ladyfingers around bottom and sides of a 9-inch Springform pan; set aside.
Beat egg whites (at room temperature) until soft peaks form. Gradually add remaining sugar, 1 tablespoon at a time; beat until stiff peaks form. Fold egg whites and whipped cream into gelatin mixture. Spoon into pan. Cover; chill 4 hours.
To serve, remove sides of Springform pan. Garnish with sweetened whipped cream.