Wash, pit, and dice the plums. Set aside.
Grease a springform cake pan with butter or coconut oil.
Separate the eggs. Beat the egg whites with a pinch of salt until stiff peaks form. Set aside.
Place the egg yolks in another mixing bowl, add the xylitol and cinnamon, and mix until light and creamy using a spoon or whisk.
Add the flour, baking powder, and water. Then gently fold in the beaten egg whites.
Pour the batter into the prepared springform pan and spread the plums evenly on top.
Bake in a preheated oven at 180 ℃ (top and bottom heat with fan/convection setting) for 20 minutes.
Once baked, remove from the oven and let the pie cool in the pan. After it has cooled completely, remove the springform ring and slice before serving.