Sift both flours, baking soda and custard powder together and set aside.
Grease a baking tin and dust it with flour. Preheat oven to 180 degrees Celsius.
In a large mixing bowl, add the milk, oil and vanilla essence. Add the sugar and mix well with a whisk till completely dissolved.
Start adding the dry ingredients little by little and keep whisking to form a smooth batter.
Now take a quarter of the batter in a small bowl. Add the cocoa powder to it and mix well, ensuring that there are no lumps.
In the baking tin drop in a ladle of the custard batter, followed by a ladle of the cocoa batter, alternating till all the batter is finished.
Use a thin sharp knife to form swirls in the batter. Take care not to mix too hard.
Place the baking tin in the center of the preheated oven and bake for 40-45 mins or until a toothpick inserted in center comes clean.
Take the cake out and let it cool in the tin for 15 mins. Transfer the cake to a cooling rack and let it cool to room temperature.