Preheat the oven to 180°C/ 160 C.
Grease 18x23cm rectangular tin with butter.
Cut the slices of sourdough in half, leaving the crust on. Generously butter one side of each slice; you can use any leftover butter later. Arrange half of the bread in the base of the dish and layer the remaining half on top.
In a large mixing bowl, whisk together the milk, eggs, sugar, cinnamon, nutmeg and orange zest.
Pour the custard mixture and any remaining melted butter over the bread and leave to soak for 5 minutes.
Wedge the slices of orange and dark chocolate chunks between the bread, leaving some to scatter on top. Sprinkle over the demarara sugar and bake for 45-50 minutes.
Allow the bread pudding to cool for 15 minutes before serving.