
In a mixing bowl, whisk the egg with the sugar until combined. Add the baking powder and about 130 g of the sifted flour, then stir to start forming a smooth batter.



Pour in the vegetable oil and warm kefir or milk. Add half of the cinnamon and mix until the batter is pourable and mostly lump-free; add the remaining flour only if the batter looks too thin.



Peel the apples if desired, then core and slice them evenly so they cook at the same rate.


Melt the butter in a nonstick skillet over low heat. Add the apple slices, sprinkle with the remaining cinnamon, and cook for about 1 minute until lightly coated and fragrant.



Pour the batter evenly over the apples. Cover the skillet and cook over low heat for 15-20 minutes, until the cake is set and a toothpick inserted in the center comes out clean. Let it rest for a few minutes before slicing and serving.






