>>>or the most authentic!
you'll need to visit the place and buy one. they do not publish their recipe and in fact claim it is illegal for anyone anywhere in the galaxy to make or sell one.
meanwhile:
Sachertorte Recipe at Epicurious.com The Sacher Torte Story Recipe: Sachertorte Sachertorte - Austrian Chocolate Cake Recipe | Apple Pie, Patis, and Pâté Sachertorte Basis Recipe Pomiane: Recipe: The Original Sachertorte...
and then I quit looking. there are zillions of recipes out there.