Peel the apples and cut them into thirds.

Arrange the apple slices in a 10-12 inch (25-30 cm) baking pan and drizzle with the lemon juice.

Sprinkle with sugar and cinnamon, then dot with the butter or margarine cut into small pieces. Bake the apples in a preheated oven at 350°F (180°C) for 15 minutes.

While the apples are baking, prepare the rest of the cake. Mix the flour with the baking powder. Beat the egg whites until stiff peaks form, and in a separate bowl, beat the yolks with the sugar and vanilla.

Gradually fold the yolk mixture into the egg whites, then slowly sprinkle in the flour mixture.

Gently fold everything together with a spatula until smooth, then spread the batter over the apples, gently swirling the pan to even it out (don't worry if it looks like a small amount—it will rise a lot as it bakes). Return the pan to the oven and bake for another 25 minutes.

As soon as you take the cake out of the oven, invert it onto a serving platter. If you like, dust with powdered sugar before serving. Enjoy!