Whisk three egg whites with a pinch of salt. When they start to foam carefully pour erythritol over them and keep on whisking until they become solid.
In a separate bowl stir together the egg yolks, a whole egg, flour and powdered sugar until all lumps have disappeared (the batter will become lighter).
Carefully fold the egg whites under the batter.
Pour the batter into a baking pan ligned with baking paper.
Bake for 20 minutes in your oven at 165°C.