Sacher cake is at its best when it is allowed to rest for a few hours to allow the flavours to develop. It can also be kept in the fridge for a couple of days.
Melt the butter and whip it with the sugar. Whisk in the egg yolks and salt.
Melt the chocolate in the microwave. Stir half a tablespoon of the butter and yolk mousse into the melted chocolate. Then you can combine the chocolate and the butter-yolk foam (without making the mixture grainy).
Mix the baking powder, wheat flour and potato flour and sift into the batter. Whisk the egg whites and gently roll them into the dough.
Pour the dough into a greased and breadcrumbed loaf pan. Bake the cake under foil at 140°C for an hour.
Allow the cake to cool before removing it from the pan. Cut the cake along the diameter from the center of the height of the cake to make 2 layers and moisten each layer with orange juice. Spread the jam between the cake and a thin layer on top of the cake.
For the icing, melt the chocolate in the microwave or in a saucepan. Be very careful as the chocolate burns easily. Add the soft butter, sugar and water. Mix until smooth and dress the cake.
Smooth the icing evenly, including the edges of the sacher cake.