Preheat the oven to 160 °C and line a 23 cm x 23 cm cake tin with parchment paper.
Make the batter. Separate egg whites from yolks.
Whisk egg whites with 1 pinch of salt by hand or electric whisker in a very clean bowl, until it gets firm, i.e. consistency is similar to whipped cream and is thick and glossy.
Whisk egg yolks, honey and 1 pinch of salt with whisker until it is foamy, its volume is doubled, and its color is light yellow.
Add whisked egg whites and baking soda to egg yolks mixture and mix very carefully. Do not break the egg whites!
Add coconut flour to the mixture spoon by spoon and mix it very gently.
Pour lemon juice and lemon zest into the batter and mix it carefully.
Pour the batter into the lined cake tin and bake for 25 minutes.
Remove the batter from the oven, smear it thinly with apricot jam.
Meanwhile make the egg whites foam. Whisk the egg whites with electric whisker. We need stiff peaks for making of this cake. Add xylitol (or erythritol) powder to the mixture spoon by spoon and mix it very gently. A stiff peak will be firm and stand straight up without bending at the tip.
Cover the batter with the whisked egg whites and sprinkle with almond flakes.
Put it back to the oven and bake for 10 more minutes, until the egg foam gets a golden color. Do not remove from the oven immediately, because the foam will collapse. Switch off the oven and leave the cake there for 20 more minutes.
Wait until it cools down, then slice with a very sharp knife and serve. Enjoy!