Preheat the oven to 180°C / 350°F and grease the 28 cm cake pan.

Peel, core, and dice the 300 g apples into small pieces.

Melt 40 g of the unsalted butter in the frying pan over medium heat.

Cook the diced apples with 30 g brown sugar and the ground cinnamon for 8 to 10 minutes, until tender and lightly caramelized.

Transfer the cooked apples to a bowl and let them cool completely.

Combine 90 g all-purpose flour, 75 g brown sugar, the seeds from 1 vanilla pod, and 80 g cold unsalted butter in a bowl.

Rub the crumble ingredients together with your fingertips until coarse crumbs form.

Whisk the 2 eggs, 125 g plain yogurt, 100 g granulated sugar, and 70 ml neutral oil until smooth.

Fold 150 g all-purpose flour for cake batter and 11 g baking powder into the yogurt mixture until no dry streaks remain.

Pour the cake batter into the greased cake pan and level the surface.

Scatter half of the cooled apples over the batter and gently swirl them into the surface.

Sprinkle half of the crumble over the apples, then add the remaining apples.

Cover the cake with the remaining crumble, distributing it evenly.

Bake the cake for 35 to 40 minutes, until golden and a skewer inserted into the cake portion comes out clean.

Heat 100 g granulated sugar in a saucepan over medium heat, swirling the pan until it becomes an amber caramel.

Add 45 g unsalted butter to the caramel and stir until fully melted.

Carefully pour in the 200 ml warm heavy cream and stir until the caramel is smooth.

Stir the seeds from 1 vanilla pod and 1 g fleur de sel into the caramel, then let it cool until warm.

Cool the baked cake for 15 minutes, then drizzle the salted butter caramel over the top before serving.