Preheat the oven to 180°C/356°F/gas mark 4.
Place parchment paper on a rectangular baking tray. Mine measures 34/24 cm. If you use a larger tray, you might want to increase the quantities but keep the ratio.
The key to a beautiful, fluffy sponge is the rule of 1, which translates into 1 egg/1 tablespoon of flour/1 tablespoon of sugar ratio. Depending on the egg's size and the type of flour, you might need extra, this is why I allowed 200 g flour, but I did not need the entire quantity. In general, I use less sugar than required, but this is a matter of taste.
In a bowl, whisk the egg whites with a pinch of salt until stiff, then leave aside.
In a different bowl (larger), whisk the egg yolks with sugar, orange zest and salt until the volume doubles. Add the tepid water gradually and keep whisking, preferably with an electric hand mixer.
Add the egg whites and mix carefully using a large spatula in a circular motion, from the bottom of the bowl to the top.
Gradually add the sieved flour together with the baking powder. Keep mixing carefully until the consistency is that of a dense cream. You might not need all the flour if enough has been absorbed to confer the desired consistency.
Pour the entire content into the baking tray. It is essential to level the batter evenly. I do this by gently slamming the tray to the worktop a few times. When happy with the result, bake for 25-30 minutes. Do not open the oven's door for the first 20 minutes!
My oven bakes quickly, so I usually reduce the temperature to 170°C as I place the tray inside.
Perform the toothpick test to check the sponge is thoroughly baked; if needed, allow extra baking time.
When ready, remove from the tray, flip upside-down on a cooling rake, and cool completely before cutting around the paper shape and dividing the sponge horizontally.