Prepare the oven and dish. Preheat the oven to 180°C/160°C fan/gas mark 4. Butter a 2-litre ovenproof dish and set aside. Bring a kettle of water to the boil.

Brown the butter. Melt the butter in a small saucepan over a medium heat. Allow it to bubble until it smells nutty and the flecks at the bottom turn golden brown. Remove from the heat and leave to cool slightly.

Prepare the orange sugar. Finely zest the orange into a large mixing bowl. Add the caster sugar and rub together with your fingertips until the sugar is fragrant and slightly damp. Add the muscovado sugar.

Combine the dry ingredients. Sift in the flour, cocoa powder, baking powder and salt. Whisk everything together until evenly combined.

Make the sponge batter. Squeeze the orange juice into a jug; you will need approximately 80ml. Whisk together the juice, eggs, milk and vanilla extract, then gradually whisk in the browned butter, scraping in all the golden flecks. Pour into the dry ingredients and stir until you have a smooth, glossy batter. Fold through the chopped dark chocolate.

Transfer to the dish. Spoon the batter into the prepared ovenproof dish and spread it evenly.

Make the chocolate sauce. In a separate bowl, mix the muscovado sugar, cocoa powder, espresso powder and flaky sea salt, breaking up any lumps. Scatter the mixture evenly over the pudding batter.

Add the boiling water. Slowly pour the boiling water over the back of a spoon across the entire surface. Do not stir. The mixture will look unusual at this stage, but this is what creates the delicious sauce beneath the sponge.

Bake. Place on the middle shelf of the oven and bake for 25–28 minutes, until the sponge is risen, set and lightly cracked, with bubbling sauce around the edges and a slight softness in the centre.

Rest and serve. Leave to stand for five minutes to allow the chocolate sauce to thicken. Spoon into warm bowls, making sure to reach the sauce at the bottom of the dish. Finish with cold pouring cream, crème fraîche or vanilla ice cream and a little fresh orange zest.