Preheat the oven to 200C/180C Fan/Gas Mark 6. Lightly grease an 8 inch square dish.
In a large bowl, combine the flour, baking powder, sugar and cocoa powder.
Make a well and pour in the melted butter, milk and egg. Whisk together to form a thick batter. Add in the zest and juice of one small orange/satsuma and the orange extract if using. Whisk until everything has incorporated.
In the dish, create a layer of orange chocolate by dividing up the segments/squares. Pour the batter over this chocolate layer.
To make the topping, in a small bowl, combine the granulated sugar and cocoa powder. Sprinkle this over the cake. Pour over the boiling water (I pour onto the back of a large wooden spoon to distribute it more evenly/with less pressure).
Bake for 30 minutes, until the top is firm. Leave to cool for a few minutes before serving (it will be piping hot in the middle/in the sauce).