The first step is to beat the 5 eggs 
 with the sugar, using an egg beater. 
You want the eggs to become double in volume, 
 fluffy, and have a withish color.  
This is an important step to make the cake moist!
The eggs are ready when they become very thick - 
you can actually create a 'ribbon' with the batter when you pour some 
 of it with a spoon over the rest of the batter!
Add the vanilla extract, a pinch of salt, and the 
 baking powder to the egg mixture.  
Continue mixing for a few seconds until well incorporated. 
Melt the butter (either in the microwave or in a sauce pan). 
Add the melted butter to the egg mixture while continuously stirring. 
Sift the flour over the egg mixture and gently fold all 
 the ingredients together using a spatula, until well incorporated.
Split the batter, equally, into 2 separate bowls
In one portion we are going to add melted dark 
 chocolate, and the other one melted white chocolate.
If you dont like white chocolate you can skip it, or 
 you can replace the dark chocolate with cocoa powder.
You can even create your own flavors, for example, strawberry 
 and chocolate, or green tea and white chocolate! 
Melt the dark chocolate and white chocolate separately.
Preheat your oven to 360 F / 180 C.
Add the melted dark chocolate to one portion of the batter. 
Then add the melted white chocolate to the other portion of the batter. 
Mix well.  
Butter your cake mold, then place parchment paper in it 
 (I am using an 8" rectangular cake pan). 
The reason we need to butter the pan before the parchment paper is to get 
 the paper to stick to the pan, otherwise, the paper will not stick! 
Cut the border of the parchment paper with scissors to get even borders.
First pour half of the white chocolate batter in the pan. 
Then cover it with the entire dark chocolate batter, 
 use a spatula to spread evently. 
Finally pour the rest of the white chocolate batter 
 on top of the chocolate batter. 
 Insert the knife in the batter and move 
 it around to create swirles.  
Bake the cake for around 45 minutes or until it is done.
 It will become golden brown, and when you press 
 the top of the cake it feels firm.
Remove the cake from the pan and let it cool down!
Bon Appetit!  
