Divide the eggs to the yolks and the whites. Put into the large bowl 300 g of a sugar and 6 yolks and mix till it is smooth and almost fluid like a dough.
As you do have the whites in small bowl, add there 1/3 of the whites and mix till it is like a snow from whites. Then add the 2/3 of whites and repeate. Add the last 3/3 of the whites and repeate (till the dough is almost like snow from whites).
Mesh 400 g of AP flour with 1 piece 13 g baking powder.
Cut the butter on a five peaces ( it shoud have been room temperature).
Add 1/5 of a butter to a mixture and mix it. Till it is well mixed. Add 1/5 of a flour with a baking powder and mix, till it is well mixed. Add 2/5 of a butter and mix till it is well mixed. Add a 2/5 of a flour with a baking powder. Mix till it is well mixed. Add 3/5 of a butter and mix again. Add 3/5 of a flour and mix still till it is well mixed. Add 4/5 of a butter and again mix. Add a 4/5 of a flour and mix. Add a 5/5 of butter and mix well. The last has to be the flour so add 5/5 of a flour and well mix. Done
Half of a dough put tu another bowl and add there 1 tablespoon of a dark cocoa. Mix the cocoa with a half of a dough.
I should have written as a first step to preheat the own on 150 degrees Celsia.
The form must be from a butter and on butter powdered AP flour. Use classical form, the big for cake.
Put to the form first half of the light dough, than all dark dough and than the second half of a light dough. Take a fork and pinch to the dough all from the up to the down around all the circle form. Pinches 0,5 cm from each other. For to get a marmor cake.
Put it into a preheat own and bake all the time on a 150 degrees Celsia.
Bake for one hour. Check it out after 40 minutes, sometimes earlier, but usually tekes an hour.
How to get the ready cake from the form? Put on the form a wet reg, it will go better. :-)