Wash, dry, and quarter the apple. Peel and remove the core. Cut each quarter lengthwise in half and then slice crosswise.

In a blender, beat the egg, about 30 g of sweetener (leave the rest for the crumble), and a few drops of vanilla flavoring until fluffy and the sweetener is dissolved (a few minutes).

Gradually add 50 g of flour (leave the rest for the crumble), yogurt, oil, and baking powder. Mix into a smooth batter (no lumps). Set aside.

In a bowl, mix flour, sweetener, and soft butter. Mix and knead to form a crumble. Add a little water if needed.

Transfer the batter to a small mold lined with baking paper or lightly greased (the cake will rise during baking, so it should not be higher than ¾ of the mold's height). Then lay a layer of apple slices on the cake. Sprinkle the crumble on top. Bake in an oven preheated to 180°C until risen and golden brown, about 50-60 minutes. Leave in the oven to cool.

Store in a tightly closed container in the refrigerator for up to 3-4 days.