Preheat oven to 350°. Grease and flour a 9 inch springform pan.
Melt the chocolate in a microwave, stirring every 15 to 30 seconds (or place in a double boiler over simmering water, stirring frequently until melted). After the chocolate has melted completely, set it aside to come to room temperature.
In a large mixing bowl, beat the butter and powdered sugar about 2 minutes until smooth. Add the egg yolks one at a time and beat for 1 to 2 more minutes. Add the vanilla and melted chocolate and beat until smooth and well blended.
In a separate clean bowl, beat the egg whites with a whisk attachment until foamy then slowly add the granulated sugar. Continue to beat until soft, shiny peaks form.
Sift the flour and cocoa together in a separate bowl.
Gently fold ⅓ of the whipped egg whites into the chocolate batter to lighten it. Next, fold in half of the sifted flour and cocoa mixture until just combined. Add another ⅓ of the egg whites, folding gently, followed by the remaining flour and cocoa mixture. Finally, fold in the last ⅓ of the egg whites until fully incorporated, being careful not to deflate the batter.
Spread the batter in the prepared pan. Bake 40 to 45 minutes or until a toothpick inserted in the middle comes out clean.
Cool for 5 to 10 minutes then loosen the cake from the sides of the pan with a knife and remove the sides of the springform pan. Transfer the cake to a wire rack and cool completely.
When the cake is cooled, slice it into three layers. It is helpful to put the cake on a turntable and hold the knife level while spinning the turntable. As you rotate the turntable, move the knife closer toward the center until you have sliced all the way through. Slide the top two layers on a plate or cardboard.
In a microwave proof bowl, heat the apricot jam and rum until it is warm and thin enough to pour. Pour the jam glaze through a strainer to get out the lumps.
Place the bottom slice of the cake on a cardboard round the same size as the cake and put it on a rack with saran wrap underneath the rack. Pour about ½ cup of the jam glaze onto the first cake layer. Allow the glaze to soak in then add the second cake layer.
Pour another ½ cup of the jam glaze over the cake layer and let it soak in. Place the top layer on the cake and pour the remaining jam glaze onto the cake. Allow the glaze to run down the sides and completely cover the cake. Use a knife to smooth the glaze into any areas that are bare. Refrigerate the cake for 5 minutes to completely set the apricot jam glaze.
To make the chocolate glaze, place the water and sugar in a medium saucepan over medium-high heat and bring to a boil. Add the chocolate and bring back to a boil. Reduce the heat to medium and using a candy thermometer, continue to heat the mixture until it reaches 230° F. You will need to stir the mixture frequently. Take the mixture off the heat and continue to stir for 1 or 2 minutes until it cools slightly.
Place the cake on a sheet pan lined with waxed paper. Pour the chocolate mixture over the top, then use a knife or offset spatula to spread it evenly, ensuring the entire cake, including the sides, is fully coated.Let the chocolate pool around the base of the cake. Refrigerate for one hour until fully set. Once firm, run a knife around the edge to separate the chocolate puddle from the cake. Carefully transfer the cake to a clean platter using an offset spatula.
Slice the cake using a knife dipped in hot water and serve with whipped cream.
Notes
Use a good quality dark chocolate. The cake is not very sweet and has a mild chocolate flavor in spite of the dark chocolate however the chocolate glaze is very sweet and chocolaty.
Just as the authentic Sacher-Torte from Vienna, this Sacher-Torte is firm and somewhat dry.
If you prefer a moister cake, thin the apricot glaze out more so it soaks in deeper.
Recipe adapted from Time Life Foods of The World Cookbook series.