Preheat the oven to 175 ° C fan oven. Peel the apples, remove the core, cut into quarters and then cut into the quarters on the back like a fan. Beat eggs and xylitol in a mixing bowl until frothy. Gradually add the butter and continue to beat until frothy. In another bowl, mix together coconut flour, nut flour, psyllium husk powder, potato fiber, baking powder and xylitol. Grease a 26cm diameter springform pan with butter and powder it with potato fibers. Now carefully stir in the dry ingredients until a homogeneous dough is formed. Now fill the dough into the springform pan and smooth it out. Spread the apple pieces on the dough with the incised side facing up and press down lightly. Sprinkle the cinnamon-xylitol mixture over the cake. Bake at 175 ° C for 50 minutes. Sprinkle with powdered xylitol to serve. With 12 pieces approx. 207 Kcal per piece.