1. Separate the eggs (make sure no yolk gets in the whites), put the egg yolks into a medium or large bowl and the egg whites into a large bowl.
2. Mix the egg yolks and 1/2 cup sugar with a fork. Sift together the flour and baking powder, add the flour mixture to the egg yolks.

3. Preheat oven to 350 degrees F (175 degrees C).
4. With an electric mixer beat the egg whites to stiff peaks. Begin at a slow speed and gradually move up to high. When the egg whites are fluffy, gradually beat in 1/2 cup sugar and continue beating until stiff peaks form.

5. Gently fold the egg whites into the yolk-flour mixture about 1/4 at a time. Don't stir or cake will not be spongy. Pour the batter into ungreased baking pan.

6. Bake the cake for about 30-40 minutes. After the first 10 minutes lower the heat to 325 degrees F (160 degrees C), do not open the oven. To check that the cake is done, insert a toothpick, it should come out clean. When done, allow the cake to cool in the open oven.

7. Once the cake is cool loosen sides with a spatula and remove from pan.
8. Meanwhile, prepare the fillings. For the first filling, combine in a medium bowl the sweetened condensed milk and melted butter, mix well with a spoon or electric mixer. For the second filling, combine in a separate bowl yogurt, sour cream and 1/2 cup sugar, mix well with a spoon or electric mixer. For a syrup to moisten the layers, mix 3/4 cup water and 3 teaspoons spreadable fruit or topping.

9. With a long knife, cut the cake into 3 layers horizontally.

10. Place the bottom layer, cut side up, on a serving plate. Moisten it with the syrup using a tablespoon or a pastry brush. Spread with the condensed milk filling.

11. Top with another layer, moisten it a bit with the rest of the syrup and spread with half of the sour cream filling.

12. Top with the final cake layer, spread the top of the cake and its sides with the rest of sour cream filling. Decorate the cake with fruits, berries, nuts or chocolate of your choice.
13. Refrigerate at least 1 hour before serving.