Put the egg whites in a bowl and whip until they form good peaks. Add the lemon juice and keep whipping - at least 5-7 minutes total.

In a separate bowl, combine the dry ingredients: flour, powdered sugar, sugar and vanillin. Sift them straight into the egg whites.

Take a spatula and gently fold the mixture until smooth, being careful not to deflate the foam.

Carefully transfer the batter into a pan greased with vegetable oil and spread it out. Tap the pan on the counter a couple of times to release excess air.

Bake for 25-30 minutes at 180 °C, until a skewer comes out dry; fresh from the oven, the sponge should be springy. Let it cool, slice and serve. Enjoy your meal!