Orange and chocolate mousse cake

SPONGE: Preheat oven to 190 ºC.
Whisk egg yolks until pale and thick with half the castor sugar, then whisk in cocoa.
Whisk egg whites until stiff and fold in remaining sugar, then fold into yolk mixture.
Spread in two 20 cm circles over baking sheets lined with baking parchment and bake for 8 to 10 minutes.
Cool on parchment, then remove and trim each sponge to fit a 15 cm diameter loose-based cake pan. Grease pan, line sides with baking parchment and drop 1 sponge into pan.
MOUSSE: Break chocolate into squares and melt dark and white separately in bowls set over hot water. Cool slightly. (Chocolate must not be too hot. If it is, it will make a greasy, grainy mixture with cream.)
Divide cream between 2 clean bowls and whisk until stiff. Fold dark chocolate into 1 bowl, white into other.
Spread half the dark chocolate mousse into base of pan, then half the white chocolate mousse. Cover with remaining sponge.
Repeat mousse layers. Chill until set.
FILLING: Stir sugar in 150 ml (3/5 cup) cold water over moderate heat until dissolved, then boil, without stirring, to make a syrup.
Add kumquats and orange slices and simmer for 10 minutes, then cool.
LID: Melt dark and white chocolate separately.
Draw a 15 cm diameter circle on baking parchment and spread dark chocolate neatly over it. Trickle white chocolate on top and leave to set.
Push cake from mould and peel off parchment.
Pile citrus fruits and gooseberries on top and set lid at a jaunty angle.
TOTAL KILOJOULE COUNT: 26 460 kJ (6 325 Cal). A portion: 1 830 kJ (700 Cal).