Put in a bowl: butter, milk, 2 tablespoons cocoa and simmer until it starts to boil, stirring gently.
When it has boiled, set the pot aside and stop in a cup, about 100 ml of this composition, mixing it with the remaining 2 tablespoons of cocoa.
Allow the rest of the composition to cool, while separating the eggs and sifting the flour. When the composition has cooled, add the yolks, one by one, then the beaten egg whites, add the flour and baking powder quenched with lemon juice.
Wallpaper a tray (mine 20x29 cm) with baking paper and pour the composition.
Put the cake in the oven heated to 220 degrees for 30 minutes. When it is ready, let it cool and pour over the icing (the 100 ml composition previously stopped). Portion, garnish as you like and serve.
From the mentioned quantities you get about 24 pieces.