Cake: Preheat oven to 190 degrees C. Grease and flour a 23cm cake tin.
Melt the chocolate in a double boiler, stir well, remove from heat and set aside.
In a large bowl beat butter, vanilla and icing sugar with a hand blender until fluffy. Gradually add the egg yolks to the mix. Stir in each yolk well before adding the next. Now stir in the melted chocolate.
In another bowl whisk the egg whites with a pinch of salt until stiff. Gradually add the caster sugar. Carefully fold the whisked egg whites into the egg-sugar mixture. Carefully fold the flour into the mixture.
Pour into the cake tin and build up the edges a bit so that the cake rises evenly.
Bake in the preheated oven for 15 minutes then decrease temperature to 170 degrees C and bake for about another 40 minutes.
When finished baking allow the cake to cool in the tin upside-down on wire rack so the cake doesn't fall in the middle.
Glaze: Mix sugar and water in a heavy saucepan. Simmer on a medium-high heat for 5 minutes. Remove from heat and stir in the chocolate gradually. The finished glaze should be thick and smooth.
Invert the cooled cake then remove from the tin.
Cut the cake horizontally in half. Brush warm apricot jam on the two halves of the cake and put them back together again.
With the smooth side (i.e. bottom side) up, place the cake on a wire rack. Stand the rack on a baking tray. Cover the smooth side of the cake with apricot jam. Allow it to soak in for a few minutes.
Pour the warm glaze over the cake. Spread it with a cake knife quickly over the top and sides. Allow the cake glaze at least 3 hours to dry.