Separate egg whites from yolks and put them into two different medium sized bowls. (How to separate egg yolk from white)
Beat yolks with a mixer on high speed. If we use any sweetener then add it now. The yolk will raise up quite a bit. It's ready when it is light yellowish in color and stops growing.
Clean beaters properly before using it to whisk egg whites. Make sure they are spotlessly clean otherwise egg whites may won't form peaks. Beat egg whites on high speed until peaks form. This will take a couple of minutes but it doesn't need to be as hard as to be able to hold a spoon on its edge.
Add the water to the yolk. Sift the flour and the baking powder with the yolks until they are completely incorporated. We can do this with the mixer without problems. We are looking for a soft mixture here. Maybe a little bit less dense than honey so if we feel that it's hard to mix then add some water to it one tablespoon at a time.
Use a spatula to fold the whites into the yolks-flour mixture little by little. Avoid stirring, we are trying to achieve a consistent yellowish light texture here without breaking the fluffiness of the egg whites too much.
Get baking form ready by covering it with baking sheet or spreading butter and sprinkling some flour on top to avoid pastry sticking to the baking form or use a silicon mold. Pour the mixture into the baking form.
Bake the sponge cake in a 356°F / 180°C preheated oven for 20 to 25 minutes. Don't open the oven's door during the first 10 minutes of baking ever! Get cake out when top is golden brown and let it cool off before slicing it up. ( Some ovens may need an hour to do this so check if it's ready by sticking a tooth pick into the pastry. If it comes out clean the cake is baked.)
Flour quality and quantity also affects the sponginess of the sponge cake. The more flour we add the denser the cake will be and less susceptible for collapsing. If our cake keeps collapsing after cooling down we need to increase the amount of flour we use to provide structure for the eggs.