Beat the eggs for 5 minutes at maximum speed until doubled in size, reduce the speed and add the sugar gradually, increase the speed again and beat for 10 minutes.
Gradually add the flour to the mixture, stirring slowly with a whisk or spatula.
Grease the bottom of a cake tin with butter, put a sheet of parchment paper over the butter and pour the mixture carefully.
Bake in a preheated oven at 180°C (356°F) for 60 minutes (do the toothpick test).