Melt the chocolate slowly (ideally in a bain-marie). Meanwhile, mix the butter with the icing sugar and vanilla sugar until creamed. Gradually stir in the egg yolks. Pre-heat the oven to 180 °C. Grease a cake tin with butter and sprinkle with flour. Whip up the egg whites with a pinch of salt, add the crystal sugar and beat to a stiff peak. Stir the melted chocolate into the paste with the egg yolks and fold in the whipped egg whites alternately with the flour. Fill the dough into the tin and bake for around 1 hour.
Remove the cake and leave to cool off (to achieve a flat surface turn the cake out on to a work surface immediately after baking and turn it again after 25 minutes).
If the apricot jam is too solid, heat it briefly and stir until smooth, before flavouring with a shot of rum. Cut the cake in half crosswise. Cover the base with jam, set the other half on top, and coat the upper surface and around the edges with apricot jam.
For the glaze, break the chocolate into small pieces. Heat up the water with the sugar for a few minutes. Pour into a bowl and leave to cool down until just warm to the taste (if the glaze is too hot it will become dull in appearance, but if too cold it will become too viscous). Add the chocolate and dissolve in the sugar solution.
Pour the glaze quickly, i.e. in a single action, over the cake and immediately spread it out and smooth it over the surface, using a palate knife or other broad-bladed knife. Leave the cake to dry at room temperature.
Serve with a garnish of whipped cream. If possible, do not store the Sacher Torte in the fridge, as it will “sweat”.