It's an airy sponge which you can add the flavor you want or add a syrup when ready. It has only 5 ingredients and it takes under 40 minutes from start to finish. The butter gives it the structure needed for layer cakes or even just to cover it with some frosting. —Francisca Azeredo Lobo
Preheat the oven to 180 ºC or 350 ºF. Grease and line a 30 X 20 cm or 12 x 8-inch cake pan.
Melt butter on low heat and let it cool.
Beat egg whites until soft peaks, with salt (optional).
In another bowl, combine yolks and sugar for about 5 minutes or until you reach an almost white and light cream.
Slowly add the butter to your yolks, beating on low.
Add a few egg whites and mix. Sift a bit of flour with the baking powder and mix.
Alternate sifting flour and baking powder with remaining egg whites, and fold very gently in between, so that you don't lose all the air from the egg whites.
Carefully, pour it into prepared pan, smooth and level the top.
Bake it for 25 minutes or until golden brown and a knife inserted into the cake comes out clean. Pay attention so it doesn't dry.
After taking it out of the oven let it sit for 5 minutes and unmold. If needed, release it from the pan with help of a knife or an offset spatula.
Let it cool on a rack.