Pour the melted butter into an 18 cm square cake pan.

Sprinkle the brown sugar evenly over it. 

Neatly layer the thin apple slices on top, creating an even caramel base.

In a blender, combine the egg, sugar, vanilla, sunflower oil, milk, baking powder, baking soda, salt, and cinnamon. 

Blend for 20 to 30 seconds until smooth and well combined.

Place the wheat flour in a bowl. 

Pour in the blended mixture and gently fold just until the batter is smooth.

Avoid overmixing as it can ruin the tenderness of your crumb.

Pour the batter evenly over the apple layer. 

Place the pan into the oven and bake for 30 minutes, or until the top is golden and a toothpick inserted into the centre comes out clean.

Let the cake rest in the pan for 10 minutes, and then carefully invert it onto a serving plate.

Allow it to cool slightly so the caramel settles before slicing and serving.