1. Preheat your oven to 350°F (180°C). Prepare the cake pan by smearing a layer of butter on it and then sprinkling some granulated sugar over the butter.

2. In a medium-sized bowl, whisk together 250g (2 3/4 cups) of flour, 6g (1 teaspoon) of baking powder, and 6g (1 teaspoon) of vanilla sugar until the dry ingredients are well combined.

3. In a large bowl, use an electric mixer to cream 225g (1 cup) of butter and 250g (1 1/4 cups) of granulated sugar until the mixture becomes light and fluffy.

4. Beat in 4 large eggs one at a time, followed by 120ml (1/2 cup) of milk, making sure each egg is well incorporated before adding the next.

5. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.

6. Divide the batter evenly between the prepared cake pans.

7. Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 5 minutes before transferring it to a wire rack to cool completely.

8. For the cinnamon crumble, combine 125g (1 cup) of flour, 3g (1/2 teaspoon) of baking powder, and 125g (1/2 cup) of granulated sugar in a small bowl. Add 60g (1/4 cup) of cold butter and mix until the mixture resembles coarse breadcrumbs.

9. For the apple filling, combine 250g (2 cups) of sliced apples, 125g (1/2 cup) of granulated sugar, and 3g (1/2 teaspoon) of ground cinnamon in a small bowl. Mix until the apples are evenly coated with the cinnamon and sugar.

10. To assemble the cake, place one of the cooled cake layers on a serving plate. Top with the apple filling, leaving a 2.5 cm (1-inch) border around the edges. Top with the second cake layer and sprinkle with the cinnamon crumble.

11. Bake for an additional 15-20 minutes or until the crumble is golden brown. Serve warm and enjoy!