Inspired by Joanna's reference to a disappointing version from Nigel Slater, I was prompted to do some research - both literary and practical......
Karl Schumacher and Eva Mayer-Bahl in "Das Grosse Buch der Osterreichischen Mehlspeisen" wail mightily about the thousands of variants on the Sachertorte that have found their way into the wild, most of which are complete nonsense and involve such things as almonds, marzipan, hazelnuts and orange rind. The version given below, they insist, is the correct recipe (translated from the German).
Real life is rarely simple, however: In Vienna, the Bakery Demel and the Hotel Sacher were locked for years in a bitter legal dispute as to who had the right to the recipe and whether or not it should have a layer of jam in the middle. One gets the feeling that there wasn't a lot to do in Vienna at the time. As in all really good silly disputes it ended in a draw. The Hotel Sacher won the right to call theirs "Original Sacher Torte" but Demel retained the right to make a cake called "Demel's Sacher Torte". Both are excellent but Demel's doesn't include the central layer of jam.
The recipe for the cake is simple, similar to making chocolate mousse. You can make the basic cake a day or two before you will need it. To get a perfectly flawless finish on the icing is a bit trickier, but worth the effort. Once you have the knack, you can ice a whole cake in a minute.
I. Making the Cake
Ingredients for a cake 24cm diameter by 5cm high: 130g Dark couverture chocolate (Felchlin, by preference, but otherwise as good as you can get); 130g Butter; 40g Icing sugar; 5g vanilla sugar; a pinch of salt; 6 eggs; 180g caster sugar;130g plain flour.
1. Melt the chocolate in a double boiler or Zimmertopf. It is important to let it cool to 35°C when you use it. You will be adding all the ingredients to the bowl you start with, so choose a large enough one to melt the chocolate in or transfer the chocolate into your work bowl once it has melted.
2. Add the softened butter, icing sugar, vanilla sugar and salt to the chocolate and beat until fluffy with a hand mixer. If the chocolate is too warm, the butter will melt and you will end up with a mess. Add the six egg yolks one at a time until blended.
3. Separately beat the six egg whites and beat in the caster sugar.
4. Fold the beaten egg whites in to the chocolate mixture.
5. Fold in the sieved flour as lightly as possible.
6. Butter and flour a false-bottomed springform baking tin - or use Trennwax spray; additionally, place a disk of greaseproof paper on the base to make unmoulding easier. Put the prepared tin onto a baking sheet. Nearly fill the tin with batter and level carefully with a straight-edged plastic scraper. Leave ¼" of rim above the batter.
7. Bake at 190°C for an hour or until a fine skewer piercing the centre comes out clean. The cake should rise but only a little higher than the tin.
8. Invert at once onto a cold baking sheet generously sprinkled with caster sugar. This ensures that both faces of the cake will be perfectly flat. When the cake is cool, score around the tin with a knife to loosen the cake, remove the base and gently peel away the greaseproof paper. Work carefully, as the cake can easily tear.
II. Assembling the Cake:
Ingredients: 350g of sieved Apricot jam - the best you can find - for the filling and coating.
1. Cut the cold cake in half across the middle.
2. Spread one half with about 1/2 of the jam and top with the other half of the cake. Ideally sit the cake on a 24cm cake disk. If your cut wasn't exactly horizontal, make sure the top goes back in the same position or the cake will be sloping.
3. Place on a wire rack. Warm the remaining jam and brush the top and sides thinly. After this stage, place the cake in the fridge - when you come to the icing stage, it will help if the surface of the cake is cold.
III. Icing the Cake (A sugar thermometer is essential for this):
Ingredients: 250g caster sugar; 120ml water; 300g grated dark Chocolate.
1. Over a low heat melt the sugar in the water; while stirring, bring to 110°C. Add the grated chocolate stir and remove the pan from the heat.
2. Continue to stir constantly and gently as the mixture cools down - it needs to cool until it is thick enough to coat the cake and not run off it completely. This will take about ten minutes for this quantity of icing. Work gently, and be careful not to stir air bubbles into the thickening icing since they will spoil the finish. If you want to test for appropriate consistency, pour a very little over the back of a coffee saucer which you've inverted over a dinner plate - you'll be able to see from the way it behaves whether or not it is yet thick enough.
3. Pour the icing over the exact centre of the cake, ideally letting only a little run off through the wire rack. If the icing is too hot it will run straight off the cake leaving a thin bumpy layer, if it is too cold it will merely sit in a blob on top . (When doing this for the first time, you had better stand the cake-rack on a tray, in case the icing flows off the cake, onto the floor and under the dog.) The perfect finish comes from letting the icing gently flow over the entire cake - you shouldn't need to touch it or smear it about. The icing that flows off can be used again if gently reheated.
Set aside to cool completely.
4. Trim the bottom edge with a knife to make it even. Loosen from the rack with a clean palette knife. It is then ready to serve, as in Vienna, with whipped cream, good coffee and the latest gossip.