Heat the oven to 175 C / 350 F. Lightly butter an 8x8 inch deep baking dish and set aside.

Zest both oranges to get 2 tsp of packed zest, then juice them to get a total of 140 ml of juice. Divide the juice: 60 ml for the batter and 80 ml for the sauce layer.

In a large mixing bowl, whisk together the flour, granulated sugar, brown sugar, cinnamon, baking powder, and salt until combined.

Add the milk, melted butter, eggs, orange zest, 60 ml orange juice, and vanilla extract. Stir with a wooden spoon until a smooth, thick batter forms with no dry streaks.

Scrape the batter into the prepared baking dish and spread it level.

In a small bowl, mix together the 100 g brown sugar and 0.5 tsp cinnamon. Sprinkle the sugar mixture evenly over the surface of the batter - do not stir.

Combine the 80 ml orange juice with 180 ml boiling water. Pour this liquid mixture slowly over the back of a large spoon held just above the batter surface, covering it evenly without breaking through.

Bake on the center rack for 32 to 35 minutes, until the top is golden and set but the center jiggles gently when you shake the dish. A skewer inserted into the sponge top should come out with moist crumbs, not liquid batter.

Remove from the oven and rest for 5 minutes before serving. The sauce will settle beneath the sponge as it stands.

Scoop directly from the dish into shallow bowls, making sure to get both sponge and sauce in each portion. Serve warm.