Separate the egg yolks from the whites and beat the whites until stiff peaks form. Set aside.
In a separate bowl, beat the egg yolks with six tablespoons of hot water.
Add sugar and the lemon peel, that you have previously grated, and keep beating until fluffy.
Add the flour and baking powder and keep beating.
Add the beaten egg whites and stop beating. Fold in with a wooden spoon from the bottom to the top until well mixed.
Preheat the oven at 180°C.
Take a 24cm round cake pan and cover it in butter and flour, so that the batter won't stick to it.
Pour the batter and spread as evenly as you can.
Cook for 40 minutes and once ready, let it cool before cutting or decorating.
Notes
Do not open the oven while cooking unless strictly necessary.
Insert a toothpick in the center of the cake and when you take it out, check if any wet part comes out. If it is dry, it's ready. You can insert it in several areas to be sure.