Beat the egg whites and sugar with a mixer on the highest level until it's really stiff. This is determined by you dipping the tip of your finger in the mixture and then it stays sticking up and does not move when you turn your finger.
Then add the oil, one yolk at a time, still mixing - but not to the maximum, and finally just mix in the flour by hand with a whisk. If you want a nutty base, add 1 and 1/2 cups of plain flour and 1/2 cup of ground nuts.
If you want to have such a perfect cake base, you have to follow the procedure to the letter in order to succeed :)
I baked it at 355 °F / 180 °C approx. 45 minutes in a round form with a diameter of 9.5 inches. After baking, I let it cool for 10 minutes and then took it out. I wiped the form with oil and sprinkled it with semi-coarse flour. It is beautifully fluffy and soft.