Once the chocolate has cooled, cream together the butter and icing sugar in a bowl until light and a little fluffy and set aside. Remove the bowl with the melted chocolate from the pan and then add the egg yolks to it and stir together with a spatula to just combine. Take the butter and sugar mixture and add this to the chocolate and egg mix and beat to combine. It might look as if it is not going to mix in at first but keep going and it will.
Once it is all mixed in, add the flour, folding it in gently with as few stirs as possible. The trick is not to over-work the mixture otherwise the gluten in the flour starts to get tough and your cake will not be super light and fluffy, although it will still taste good.
In a separate bowl add the egg whites, and using a hand-held electric whisk or a stand mixer fitted with a whisk attachment, whisk a little until frothy. Then add one quarter of the caster sugar at a time, whisking well between each addition, making sure that all of the sugar is dissolved before you add the next lot. Once all of the sugar has been added, keep whisking it until the mixture becomes firm and shiny. Some people say to hold the bowl over your head – if it doesn’t fall out, it’s ready!
Tip the meringue into the chocolate mixture and gently fold everything together, making sure it is all combined. Divide the mixture between the lined tins.
Place a cooling rack upside down on top of both of the cakes. Holding onto both of the cakes and the cooling rack, flip everything over so that the cakes are now upside down on the cooling rack. Carefully remove the tins and then leave them to cool down like this completely – it will mean that the cakes are nice and flat.
When the sponges are cool, prepare the filling by gently heating the apricot jam in a small pan or in the microwave so that it is spreadable. Spread a sponge with warmed apricot jam, and then sandwich the other one on top. Place the cooling rack into a deep baking tray (to catch the excess chocolate glaze) and set aside.
To make the chocolate glaze, heat the cream in a pan over a medium heat until almost boiling. Then take it off the heat and add the dark and milk chocolate along with the sugar. Let it sit for a few moments and then using as few stirs as possible, stir it through to check that everything is melted.
When the chocolate has melted, add the butter a little at a time, stirring well between each addition until the glaze has thickened and is nice and shiny This whole stage needs to be done quite quickly as the chocolate is ready when it reaches just body temperature. To test this temperature put a little on your lower lip and it should feel really warm but not hot to the touch.
I pour it over the middle of the cake to start with and find that the more you pour it, the force pushes the chocolate glaze over the edges of the cake and down the sides. You may have to pour the chocolate around the edges at the end to make sure all bits are covered.
Leave the Sachertorte to set at room temperature because if you put it in the fridge it can dull the beautiful mirror-like glaze, and then serve.