200 g butter, at room temperature, plus extra for greasing
4 tablespoons cocoa powder
3 large oranges
200 g caster sugar
3 large free-range eggs
200 g self-raising flour
1 teaspoon baking powder
200 ml double cream
For the chocolate topping
100 g butter
100 g good-quality dark chocolate (70% cocoa solids), broken up
100 g icing sugar
Preheat the oven to 180°C/350ºF/gas 4. Line the bases of two 20cm/8 inch sandwich tins with greaseproof paper, then grease the sides of the tin with a little of the butter.
Add 4 tablespoons of boiling water to the cocoa powder and mix until smooth. Grate the zest of 2 of the oranges. Beat the sugar and butter until fluffy, then add the cocoa mixture, eggs, flour, baking powder and the orange zest, and mix well.
Divide the mixture between the sandwich tins and bake in the oven for 20 to 25 minutes. To check the cakes are cooked, stick a skewer into the middle of one and leave it for 5 seconds. If it comes out clean, the cake is ready. Allow them to cool for 10 minutes, then remove from the tins and place on a cooling rack.
Meanwhile, place the chocolate topping ingredients in a heatproof bowl over a pan of lightly simmering water. Make sure the water isn’t boiling and the bottom of the bowl doesn’t touch the water, or you will burn the chocolate. Zest the remaining orange, putting the zest to one side, then cut the orange in half and squeeze 3 tablespoons of juice into the bowl. Stir until melted and mixed, then allow to cool. Whip the double cream until you have soft peaks. Segment the two other zested oranges with a small, sharp knife.
Remove the greaseproof paper from both the sponges and place one on a plate or cake stand. Spread the cream over the sponge and place the orange segments on top. Sandwich the second sponge on top and press down slightly. Drizzle the chocolate topping over the cake and leave to firm slightly. Decorate with the remaining orange zest.