Preheat the oven to 355 degrees F/180 degrees C. Using an egg separator, separate the egg whites from the yolks. Sift the flour with baking powder and set aside. Butter an 8-inch/20 cm round cake pan and line it with parchment paper (you can also use a Le Creuset springform pan of the same diameter).

To make the batter, place egg whites with a pinch of salt in a large bowl and beat with an electric mixer until foamy. Add sugar in three batches and beat until stiff and glossy. Then reduce the speed of the hand mixer and mix the yolks, oil, and sifted dry ingredients into the mixture. Mix to combine the ingredients; avoid overmixing the cake mixture. Pour the batter into the prepared pan.

Peel, core, and slice the apple into 1/2 inch/1 1/2 cm slices. A vegetable peeler and an apple corer make this job easier. Put apple slices on top of the batter, slightly pressing them.

Bake the cake for 40-45 minutes, until a toothpick inserted in the center of the cake comes out clean. Take the cake out of the oven, and rest it in the pan for 10 minutes. Remove the cake from the mold, unpeel the parchment paper from the sides, and let it cool on a wire rack.

 Once cooled, transfer it to a serving plate and sprinkle with powdered sugar.