In a bowl, work the butter into a cream, add the salt and sugar, then the egg yolks one by one. Add the flour, a spoonful at a time, then a whole egg. After a short time add another yolk. When the mixture is very smooth, incorporate the stiffly beaten egg whites.
Cover a baking tray with greaseproof paperand pour the mixture in, making sure it is no more than half a centimetre thick. Leave it to cook in a preheated oven at 180° for about 20 minutes.
When the top starts to brown, take it out the oven. Let it cool and then cut it into even rectangles.