Preheat the oven to 180°C. Prepare a baking tray 40x28 cm with baking paper on it.
Start by whisking egg yolks and sugar until creamy. When the mixture is fluffy, add warm water one tablespoon at a time while whisking the mixture.
Add sifted flour and baking powder. Mix well.
In another bowl whisk the egg whites with a pinch of salt. When the egg whites are whipped, fold them in gently.
Pour the batter on the baking tray and add small cubes of chopped and peeled rhubarb on the top. Bake for 25 minutes.
Let the sponge cake cool completely.
