from the Sacher-Kochbuch, sort of
There are as many recipes for Sachertorte as there are cooks who make it, likely, even in its native Vienna. Legend has it that one day during the Congress of Vienna (1814 - 1815, when Vienna was hosting the rest of Europe and spending everything it had and more on impressing them, which largely worked), Kaiser Franz Joseph asked that a special cake be made him. However, that morning, his regular pastry chef was ill (on vacation? gone fishing?), and a young protégé Franz Sacher dreamed up a new confection in his stead, mostly chocolate, but lightish, and with a layer of jam underneath the topmost frosting. For some unknown reason, Franz Sacher later sold his original recipe to Demel's, a fancy café on the Kohlmarkt, while the Hotel Sacher, on Philharmonikerstraße, made its own variant, calling it the "Original Sachertorte," while Demel's has to call its "Demel Sachertorte." Meantime, every Kaffeehaus in the city has its own Sachertorte, no two quite alike, but all excellent.
I'd love to put a stop to the debate and present a definitive recipe for Sachertorte, but I can do no such thing. In fact, I'm not even sure I can present any recipe, as every one I've seen so far uses the metric measurements -- which, sensible as they may be in most things, are useless for cooking for my purposes. Below is an attempt metric-to-cups-&c. conversion, instead -- sorry if the measurements are a little ridiculous. It is, however, from the Sacher-Kochbuch, which sounds pretty definitive to me!
Slightly warm the butter and whip with powdered sugar and a touch of vanilla extract until foamy. Separate the eggs, and mix in the yolks (slowly) and pre-warmed chocolate. [NOTE: Since they want less than 5 oz chocolate here, and a little more than 5 oz for the icing, melt 5 oz here, but take out one of the square before it's fully melted, and save it for the frosting. A little extra chocolate in the frosting won't kill anyone, except your dog, so don't feed it to your dog.] Whip the egg whites almost stiff, then whip in granulated sugar to full stiffness and carefully add the whites to the dough.
Equally carefully, add the flour. Put it in a pan -- springform? (probably not necessary), and bake at 338 degrees F, keeping the door of the oven slightly open (about 1 inch) for 12-15 minutes, then bake with the door closed for 1h. [NOTE: That's ridiculous! Maybe just keep the door closed ...]
The cake is done when you get a slight "response" upon touching the surface with your finger. Now let the cake cool in the form, first for 20 minutes turned upside down, then the right way round until it is completely cooled. Only then you should take the cake out of the form.
Now halve it horizontally and cover the top and bottom of each half lightly with pre-warmed apricot jam, put the halves together and cover the sides of the cake with jam. [NOTE: I never saw jam in the middle, just on the top and sides. When you do this, make it a very thin layer, and try not to have any lumps.] Let dry and put chocolate icing on.
Boil the sugar in the water for 5-6 minutes and let cool partially, then add the softened chocolate [NOTE: adding the chocolate bit from before that you saved] and mix thoroughly until you get a viscous, homogeneous mass. For the icing, it is important that the chocolate mass has the right temperature before you apply it to the cake -- the Austrians call it "lip-warm." If the temperature is too high, the cake won't glisten and if it is to low, the glazing will not dry well. The cookbook says that if you let a little bit of chocolate run over the backside of a spoon, approximately 4 mm of icing should remain. Then the temperature is correct.