1 Grease a two-litre oven-proof baking dish and preheat the oven to 180 degrees, or 160 degrees fan.
2 Place the chopped chocolate in a heatproof bowl over a saucepan of simmering water and stir regularly until the chocolate is completely melted. Alternatively, melt the chocolate in the microwave in 30-second increments to ensure the chocolate doesn't burn. Once melted, set aside to cool slightly.
3 Meanwhile, in large bowl, sift together the self-raising flour, cocoa powder, baking powder and salt. Add the light brown sugar and mix well.
4 In a separate bowl, grate in the zest of the two oranges and squeeze in the juice from both oranges. Add the cooled, melted butter and whisk in the eggs. Add the milk and whisk well to combine. Add the Cointreau orange liqueur (if using). If you would prefer to leave it out, substitute with another 50ml of freshly squeezed orange juice.
5 Pour the wet mix (the eggs, milk, butter and orange juice) into the dry (the flour, cocoa, salt, baking powder and sugar) and mix well until smooth.
6 Pour in the melted chocolate and whisk to combine. Pour the pudding mix into the buttered dish and smooth the top using a palette knife or the back of a spoon.
7 Make the sauce by whisking together the brown sugar, cocoa powder and boiling water until smooth. Slowly pour this mix straight on top of the pudding mix. Don't worry if it looks like there are lumps, it will even itself out in the oven.
8 Bake the pudding in the preheated oven for 35-40 minutes. The pudding will have puffed up a little and look beautiful and luscious.
9 This pudding is best served warm, so leave to cool for about five minutes before serving. Scoop spoonfuls of pudding on to each plate and drizzle a little of the sauce that has pooled to the bottom on top. Serve with crème fraîche, cold cream or your favourite ice-cream.