How to separate the egg yolks from the whites: Crack the egg and let the egg white fall into one of two clean bowls you’ve laid out. Slowly and carefully open the cracked egg so that the yolk stays inside, and the whites come out. Pass the egg yolk back and forth from one half of the shell to the other, while allowing any remaining egg white to seep out into your bowl. After you’ve gotten all of the whites out of the shell, you can place the yolk in the empty bowl;
Melt the butter in the microwave and let it cool down; then add it to the egg yolks and the sugar. Be careful—if the butter is hot, you will cook the yolks, and your batter will turn into a sweet frittata;
Stir all the ingredients until well combined; you may also use the dough-kneading hook on your KitchenAid;
Sift together the flour, cornstarch, and baking powder so that they are evenly combined. Add it to the egg mixture little by little to avoid any lumps. I like to use a silicone spatula as it makes an even and smooth batter;
Whip the whites with the mixer at high speed until they form stiff peaks;
Peel, core, and slice the apples. If you use a hand-crank apple peeler, the slices will be perfect and you won’t waste any parts of the apple;
Fold the egg whites into the batter with a downward motion to maintain as much volume as possible. The operation should be done quickly so as to prevent the apple slices from oxidizing;
Transfer the batter to a pan lined with parchment paper and on the sides as well so that you can remove it easily. This eliminates the need for any cooking sprays;
Top the batter with the apple slices;
I usually place them at the right angles so that when the batter rises, the outer part of the slices will stay on the surface and the inner ones will be buried in the cake batter. Sprinkle the top with raw cane sugar for a delicious crunchy crust;
Pre-heat the oven to 340°F (180°C) and bake for 40 minutes on the middle rack of a static oven;
If the top gets too brown, lower the temperature to 320°F (160°F);
Do the toothpick test to check for doneness. If it comes out clean, it is ready. If not, bake the cake for a few more minutes until the toothpick comes out clean;
Leave the cake in the turned-off oven for about 5-7 minutes so that it cools down gradually; then open the oven door and let it rest another 10 minutes;
If you bake it the day before, the taste will be even more flavorful.