Preheat the oven to 170° top and bottom heat. Grease a loaf pan (20 cm long) with melted butter so that the dough does not stick.
Peel the apples and cut them into small cubes.
Put the butter in a bowl and beat it with a mixer until fluffy. Important: It should be room temperature, otherwise it is difficult to beat.
Separate the eggs and add the yolks to the butter. Add erythritol and stir until everything is nice and fluffy.
Beat the egg whites until stiff.
Mix the grated almonds and cinnamon. Fold the mixture into the butter-egg mixture with a dough scraper.
And now fold in the egg whites and the apple pieces.
The cake takes about 40 minutes. Do the test with chopsticks. If it is not yet done, cover it with baking paper and put it in the oven for another 5-10 minutes.