Preheat oven to 300�F / 150�C and prepare mold by smearing butter in it. 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 until light yellow. Addu00a0 sweetener and orange and lemon zest to the beaten yolks. Clean beaters properly before using it to whisk egg whites. Make sure they are spotlessly clean otherwise egg whites may wonu2019t form peaks. Beat egg whites on high speed until soft peaks form. This will take a couple of minutes but it doesnu2019t need to be as hard as to be able to hold a spoon on its edge. Add corn starch to the beaten whites. It will support the structure of it. Alternating between water and flour add them to the yolks. 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 itu2019s 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. Pour the mixture into the mold and put it into the preheated oven. Bake it for about 50 minutes or until toothpick comes out clean. Donu2019t open the ovenu2019s door during the first half an hour of baking ever! Get cake out when top is golden brown. After taking it out of the oven flip it onto a cooling rack and let it cool down before slicing it up or it could collapse quite badly.