Cake: melt the chocolate and set it aside. Mix the butter with 100 g of sugar in a bowl. Blend in the egg yolks (one at a time), the melted chocolate and the flour, sifted with the salt and baking powder. In another bowl, whisk the egg whites until stiff with the rest of the sugar, then add them to the mixture. Fold in carefully, stirring from the bottom up. Turn the mixture out into the buttered and floured cake tin, levelling the surface.
Bake the cake in the oven preheated to 175°C for 42/45 minutes.
When done, remove from the oven, turn out and leave to cool on a grid.
Filling: cut the cake in half to form 2 discs, then coat the bottom half with the apricot jam.
Put the cake back together and brush it all over with the hot apricot jelly.
Icing: melt the chocolate with the butter in a hot bain-marie and set aside.
Heat the icing-sugar and water until the syrup is simmering, then pour in the melted chocolate and butter and return to the boil.
Take off the heat at once and whisk the icing briskly until it is velvet-smooth, then cool to room temperature.