Place the eggs in a bowl with the sugar and a pinch of salt, beat with an electric mixer, or a whisk, until you get a clear and fluffy mixture
Add the butter that has been melted in a bain-marie and left to cool, then the milk. Continue to mix with the help of a spatula and finish by adding the previously sieved flour and yeast.
Cream until you obtain a homogeneous mixture. Take 1/3 of the batter, pour it into another bowl and add the bitter cocoa that you have dissolved in 1 cup of water (coffee cup), mixing well.
Pour the white mixture into a mould that has been previously buttered and floured, then pour the cocoa mixture and pass the blade of a spiral knife between the two mixtures in order to obtain the marble effect. Bake in a preheated static oven at 180°C for about 40/45 minutes.
Remove from oven, let it cool down and decorate with icing sugar.