Beat egg whites: Shortly before the baking time for the apricots is up, separate the eggs. Beat the egg whites until stiff. Add 80 g/ 2.8 oz/ 1/3 cup + 1 tablespoon sugar and the lemon juice and continue beating until the egg whites are stiff and glossy. Set aside.
Remove the cake from the oven. Leave to stand for 3 minutes.
Make the batter: In the meantime, carefully fold the egg yolks into the egg white mixture. Use a hand whisk to do this, and don't overdo it. Next, add the mixed flour and baking powder over the egg mixture and fold in very carefully again; you should not overdo it either.
Bake: Spread the mixture over the fruit, level nicely and bake for about 30 minutes. Don't open the oven during the first 20 minutes of the baking time. Check after 20 minutes and reduce the temperature to about 160 degrees Celsius/ 320 degrees Fahrenheit if you think the cake already has too much color.
Rest: After the baking time is over, take the cake out of the oven. The cake is ready when a toothpick inserted in the batter layer comes out clean. Leave to stand for 5 to 10 minutes.
Turn cake on a platter: Then, run a knife around the edges of the cake to release it correctly from the walls of the baking dish. Next, place a rectangular serving dish on the baking dish and reverse the cake onto the serving dish.
Tip: At first glance, you might think that the cake is undercooked because the butter-sugar layer will look rather wet. But it is not undercooked or wet; it is just glossy because of the butter and the sugar.
Cut the cake into squares and enjoy warm or cooled with some whipped cream on the side, if you wish.