1. Preheat oven to 180°C (350°F).

2. Grease a 12-cup muffin tin thoroughly.

3. In a large bowl, whisk together flour, baking powder, and ground cinnamon.

4. In another bowl, beat eggs, granulated sugar, sour cream, and vanilla sugar until smooth.

5. Add melted butter and mix until fully incorporated.

6. Gradually fold the dry ingredients into the wet mixture until just combined.

7. Divide the batter evenly among the muffin cups, filling each about 2/3 full.

8. Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean.

9. Let the cakes cool in the tin for 10 minutes, then remove and cool completely on a wire rack.

10. While cooling, prepare the filling: in a small saucepan, combine diced apples, 1.5 tablespoons sugar, cinnamon, lemon juice, and 1 tablespoon butter.

11. Cook over medium heat for 5–7 minutes, stirring occasionally, until apples are softened but still hold shape. Remove from heat and let cool.

12. Using a small round cutter or serrated knife, cut a lid from the top of each cooled cake.

13. Spoon a portion of the apple filling into the center of each cake.

14. Replace the tops and serve slightly warm or at room temperature.