Chocolate orange cake

Name: Chocolate orange cake
Ingredients: 75g/2¾oz cocoa powder; 6 tbsp just-boiled water; 2 medium oranges, well-scrubbed, finely grated zest and 2 tbsp juice only; 225g/8oz unsalted butter, cubed and softened, plus extra for greasing; 225g/8oz caster sugar; 4 large free-range eggs; 225g/8oz self-raising flour; 1 tsp baking powder; ½ orange, well-scrubbed; 25g/1oz caster sugar; 300ml/½ pint double cream; 300g/10½oz dark chocolate (about 70% cocoa solids), broken into roughly 1cm/½in pieces (see recipe tip); 2–3 tbsp semi-skimmed milk