Sieve the flour and cocoa together and put them aside.
Separate the yolks from the egg whites, keep the egg whites aside.
Pour the yolks at room temperature into the mixer bowl with a whisk hook and operate it at medium-high speed, adding the sugar little by little.
After about 5-6 minutes the mixture will become clear and frothy, then add the cold water, vanilla seeds, and salt.
At this point also pour the warm melted butter and mix again for a few moments, then lower the speed to the minimum and add a spoonful of flour and cocoa which were previously sieved together.
After incorporating the powders, add the heated milk, which should be heated at 40C.
Beat the egg whites with the electric whisk in a separate bowl, adding the lemon juice:
the egg whites must be semi-whipped.
Pour the beaten egg whites into the bowl with the cocoa mixture.
Incorporate the egg whites evenly with a spatula.
The final compound will be very fluid, almost liquid.
Pour the dough into the previously lined baking pan (9x9 inches) and bake in a preheated static oven at 150 °C (300°F) for about 70-80 minutes.
Check the consistency with a toothpick to make sure that it’s still not liquidy.
Remove the cake from the oven and let it cool completely.
Put it in the fridge for at least 2 hours.
Remove the mold and cut it into cubes and sprinkle with icing sugar.