Preheat the oven to 180°C /356°F.
Grease and line the loaf tin with baking paper/parchment paper.
In a small bowl, add cocoa powder, milk and mix well. Keep aside.
Take a bowl, sift plain flour and baking soda. Keep aside.
In a mixing bowl of a stand mixer attached with a paddle hook, beat eggs along with sugar until light and fluffy. It will take approximately 5-7 minutes.
Then add the melted butter, milk, vanilla extract and beat for 1 minute on slow speed.
Next add the sifted flour, baking soda along with salt and slowly beat until they are combined well.
Divide the batter equally into two bowls. In one bowl, add the cocoa powder mixture and combine everything well.
Take the greased loaf tin, first transfer the half of vanilla batter and then add the chocolate batter and finally add remaining vanilla batter.
Using a Barbecue stick or knife swirl the vanilla batter into chocolate batter to create a marble effect.
Bake in the preheated oven for 30-40 minutes or until the skewer comes clean (here I mine done at 38 mins)
Allow it to cool in the tin for 10-15 mins and then transfer it on a wire rack to cool completely.
After that slice the delicious and moist cake and enjoy with a cup of tea or coffee.