Preheat oven to 325°F (163°C). Line an 8 x 8 inch baking pan with parchment paper.

Cover the bottom of the pan with a single layer of apple slices, lining them next to each other side by side. Add a second layer of apple slices on top, overlapping them slightly with the first layer so that the second layer of slices fills the gaps left by the first layer. See photo in post for reference. Only use as many apple slices as needed to create two layers. You will likely have some leftover slices.

In a large mixing bowl, add egg whites. Beat egg whites on highest speed with a mixer until stiff peaks form. Set aside beaten egg whites into the fridge until ready to use.

In a larger mixing bowl, add egg yolks, flour, sugar and ½ cup of milk. (Make sure to only add ½ cup right now. If you add all of the milk you will not be able to mix your batter.) Beat on high speed until batter is smooth and no flour lumps remain. Once batter is smooth, gradually add in remaining milk (1 ½ cups). Reduce speed on mixer and mix until milk is incorporated into the batter. Your batter will be very thin.

Remove egg whites from fridge. Fold in ⅓ of the egg whites into the cake batter with a spatula. Because the cake batter is so thin, you do need to swirl and fold to make sure the egg whites are fully incorporated. Make sure to fold all the way from the bottom of the bowl, otherwise the liquid batter at the bottom won't get fully mixed with the egg whites. Fold until no egg white streaks remain. Repeat with the next ⅓ and then final ⅓. For the final ⅓, you do not need to mix until no egg white streaks remain. It's fine to have some very small lumps of egg whites in your batter. Your batter should look very light and airy.

Pour batter into the prepared cake pan over the apple slices. Bake for about 40-50 minutes, or until cake is done (mine took 45 minutes). If you are unsure if your cake is done, it is better to bake a little longer (5 minutes or so) to make sure it is done so that your cake does not collapse. When the cake is done, a cake tester should come out clean. The top should be evenly browned. Let cake cool completely before cutting. As the cake cools it will deflate a little.

Let cake cool completely before serving (at least a few hours or overnight). If desired, dust top of cake with powdered sugar before serving. Cake can be served the same day or the next day. If serving the next day, store cake in fridge in an airtight container. Then let it come to room temperature 2 hours before serving.