Preheat the oven to 350°F(177°C). Grease 8 ½ * 4 ½ inch (21,5*10,5cm) loaf pan and line the bottom and sides with parchment paper, about 2 inches(5 cm) of overhang on each side. If you like, you can grease and flour the dish instead of parchment paper.
Place the flour, baking powder and salt if using in a medium bowl, set aside.
Put the eggs and sugar in a medium bowl and stir with a whisk for almost 30-40 seconds until just combined.
Add in the milk, melted butter and vanilla extract, stir with a whisk.
Add the flour mixture in 3 batches. You can stir with a whisk and then continue with a spoon as it makes this step easy.
Divide the batter almost in half: either dip 1 cup into the batter and place it into a medium bowl without leveling, or after you divide the batter in half, add extra 2 tablespoons from the batter and place it into the other bowl without leveling. Approximately one batter will be around 320 grams, the other batter will be 420 grams. If you like you can divide the batter in half but be aware that you'll see more chocolate parts once sliced.
Give a stir to the cocoa powder to avoid lumps and add into the bowl that is less in batter, stir with a spoon.
Spoon each batter, alternating both horizontally and vertically, into the pan. There'll be two rows. If you start with chocolate batter, then continue with vanilla and then chocolate again. The chocolate batter will be thick so you can use two spoons if you prefer. When you come to the next layer, put the opposite batters on top of the first layer and do the same for the third layer. Using a knife, make a swirl from the beginning to the end of the pan.
Bake for 40-42 minutes until or until a toothpick comes out clean. There can be few moist crumbs which are fine. You should check your oven after 35 minutes as the baking time may change from oven to oven.
Let the cake cool for 20 minutes in the pan and 20 minutes on a wire rack before slicing.