Prepare the Oven and Pan: Preheat the oven to 180°C / 350°F. Line or grease a 20 cm / 8-inch brownie pan.
Prepare the Oranges: Zest and juice the oranges, measuring the required 2 teaspoons of zest and 120 ml (½ cup) of juice before beginning the batter.
Melt the Chocolate and Butter: Place the finely chopped dark chocolate and unsalted butter in a heat-proof bowl. Melt them over a double boiler or in the microwave using 30-second increments, stirring as needed, until the mixture is smooth. Set aside to cool. Whisk the Eggs and Sugars: In a separate large bowl, whisk together the brown sugar, caster sugar, room-temperature eggs, and orange zest until the mixture is slightly bubbly. Do not whisk for longer than necessary, as incorporating too much air can make the brownies rise more and become less fudgy. Add the Orange Juice: Pour the measured orange juice into the egg-and-sugar mixture and mix until combined.
Incorporate the Chocolate Mixture: Add the cooled melted chocolate and butter mixture to the bowl, mixing until the ingredients are evenly combined.
Add the Dry Ingredients: Sift the flour and fine salt into the wet mixture. Mix only until the flour has just been incorporated, taking care not to overwork the batter.
Fill the Pan: Pour the finished brownie batter into the prepared 20 cm / 8-inch pan and spread it into the pan.
Add the Orange Topping: If using, thinly slice an orange and cut each slice in half. Arrange the orange pieces over the surface of the brownie batter.
Bake the Brownies: Bake at 180°C / 350°F for 20–25 minutes, or until the top is set and shiny.
Cool Completely: Remove the brownies from the oven and allow them to cool completely before taking them out of the pan and slicing, as they will be very soft while warm and can break apart if moved or cut too soon.
