For the filling, mix together lemon juice, cinnamon and sweetener. Peel, quarter and core the apple and cut into very small cubes. Mix the apple cubes with the marinade.
For the dough, beat the eggs, vanilla sugar, sweetener and salt until frothy. Gradually add the oil. Mix both types of flour with the baking powder and alternately add portions to the egg mixture with the buttermilk.
Briefly drain the marinated apple cubes in a sieve and stir 3/4 of them into the batter. Pour the batter into a greased muffin tin. Spread the remaining apple pieces on the dough.
Bake in the preheated oven at 180 ° C for about 25 minutes.
After baking, let the muffins rest for about 10 minutes, then remove them from the mold and leave to cool on a wire rack.