Preheat the over to 170°C/375°F/gas mark 5.
In a large mixing bowl, place the eggs at room temperature (this is important as it allows the volume to increase when battering!), sugar, salt and vanilla. Batter with an electric mixer for 5-7 minutes until the quantity triples.
Add the water gradually and mix gently with the spatula. You might not need to use the entire quantity of water.
Gradually add the sieved flour mixed with baking powder. Only use as much as needed to obtain a medium thick batter that still flows down easily. Different flours absorb humidity differently. If the batter becomes too thick and does not flow nicely from the spatula, try adding a tablespoon of water.
Use a large spatula to mix gently, from the bottom of the bowl up, in a circular motion to help keep the fluffiness of the batter.
Pour the batter over the apples placed in the baking tray and allow it to spread evenly. It should cover the apples, and it will rise while baking.
Preferably, cover the tray and bake for 20 minutes, then remove the cover and bake for the remainder of 20-25 minutes. I use a shallow pizza tray as a cover to prevent the burning of the cake's top.
The baking time is longer for this cake as the apples need extra time to bake.
Perform the toothpick test to check that the sponge is baked thoroughly and allow extra time if needed.
When you are happy that the cake is ready, remove it from the oven and place the tray on a cooling rack.
Allow cooling for about 10 minutes. Meanwhile, prepare the cake stand you will use for your finished cake.