In a bowl, mix butter ( slightly melted ) with sugar.
Once the butter and sugar are well integrated, add egg yolks and stir with a whisk until everything is well integrated.
Add milk and continue stirring with the whisk.
Gradually add dry ingredients: flour and baking powder. Mix everything with a whisk until there are no lumps.
In a separate bowl, beat the remaining egg whites until stiff.
In batches, add the meringue to the mixture and combine with a spatula with slow folding movements until well integrated.
Separate the cake batter into two bowls
In one bowl put 2/3 of the mixture and in another 1/3 of the mixture, to the latter add a tablespoon of cocoa powder and stir well.
Spread butter on the base and sides of the loaf pan, then sprinkle flour all over the pan.
Pour half of white batter into the loaf pan, then add all the cocoa batter and finish by adding the rest of the white batter.
Bake for 50 minutes. When the time is up insert a cake tester (or a knife) into the center of the marble cake to check whether it is done. If the tester comes out clean … Et voilá! Your delicious and fluffy marble cake is ready!