Break up the chocolate and place it in a heatproof bowl.  Set the bowl over a saucepan of gently simmering water, making sure the steam doesn't touch the chocolate and the bottom of the bowl doesn't touch the water.  Stir frequently until it is melted, then remove it from the saucepan to cool slightly.

Beat the butter in the bowl of a stand mixer fitted with the paddle attachment until smooth, then add the sugar and beat until light and fluffy.

Scrape down the sides of the bowl with a flexible spatula, then mix in the egg yolks, vanilla extract, and salt until well blended.  Scrape the bowl again, then stir in the chocolate.

In a clean, grease-free bowl, whisk the egg whites until stiff enough not to fall out of the bowl if it's turned upside down.  Don't overbeat, or the egg whites will look dry and lumpy.

Energetically stir ⅓ of the egg whites into the mixture to slacken it, then gently fold in the remaining egg whites until flecks of egg white remain.  Sift the flour on top of the mixture, then continue folding until the mixture has no lumps of egg whites or streaks of flour anywhere.

Gently pour the batter into the prepared pan and tap it on the counter a couple times to remove any large air pockets.

As soon as the cake comes out of the oven, run a thin knife around the edge and turn it out onto a flat surface and let it rest there for 10 seconds or so.  This will help flatten the domed top.  Then, turn the cake right side up and place on a wire cooling rack until completely cold, approximately 1 hour.

Once the cake has cooled completely, carefully slice off the domed top with a serrated knife, then slice it in half to create two layers.

Place the apricot preserves in a small saucepan over medium heat.  Stir until warm and melted, only adding a little water if the preserves are too thick.  Press the preserves through a sieve with a spoon to remove the lumps of fruit.

Spread about ⅓ of the preserves on top of one of the cakes, then place the other layer on top.  Pour the remaining apricot glaze on top and smooth over the top and sides with an angled spatula.  Leave to set completely.

Pour the heavy whipping cream into a small saucepan set over medium heat.  Heat until steamy and just barely bubbling, then immediately remove from the heat and dump in the dark chocolate.  Whisk until smooth.  Let the ganache cool for a minute or two until slightly thickened, but still pourable.

Place the cake on a wire cooling rack set over a cookie sheet.  Pour the ganache evenly over the top and sides, allowing the excess to drip off the rack onto the cookie sheet below.  Leave to set completely.  Your finger should not stick to the ganache when it's set.

Before the ganache is completely set, melt the milk chocolate chips in a heatproof bowl set over a saucepan of simmering water, stirring frequently.  Pour the melted chocolate into a plastic sandwich bag, twist the top, and snip off the corner to create a mini piping bag.  Practice piping the word "Sacher" on a paper towel, then pipe "Sacher" on top of the cake to decorate.  Leave to set completely.