Preheat the oven to 180'C.
Melt the chocolate in a heatproof bowl over simmering water (bain-marie).
Zest and juice the orange. Reduce the juice with half of the sugar until you have a syrup.
Stir into the melted chocolate.
Sift the flour, baking powder and cocoa powder together. In a bowl, beat the sugar and the butter together until the mixture is pale and fluffy. Beat the eggs into the butter mix one at a time. Stir in the sunflower oil and then gently fold in the flour. Finally, add the melted chocolate.
Pour the mixture into a cake tin and bake for 30 to 40 minutes. After about 30 minutes check the cake is cooked by inserting a skewer into the centre. If it comes out clean it is ready. Allow to cool and then transfer to a cooling rack to ice the top of the cake.
Place the cream and the golden syrup into a saucepan and bring to the boil. Whisk in the cocoa powder. Bring back to boil and simmer for 3 minutes. Allow the mixture to cool slightly and then pour it over the cake. Make sure the cake is cold before pouring on the icing otherwise it will melt.
«The chocolate icing should be cold enough before pouring over the cake but still runny. The consistency should be of a thick batter.»