We will start by preparing a baking tin. Take a loaf tin (either 9x5 inches or 8.5x4.5 inches) or a round baking tin (8 inches). Grease it with oil or butter. Use 1 tbsp flour (maida) to dust the pan thoroughly. If need be, cut a piece of parchment paper to fit the bottom of the pan and stick it over the greased surface.
Preheat the oven at 180C.
Into a large bowl, sieve the flour, baking powder and salt.
In another bowl, mix the oil, sugar and eggs. Beat this with an electric handheld whisk for 3-4 minutes until pale yellow and fluffy. If you don’t have an electric whisk, then use a regular whisk, but spend some time aerating this mixture well.
Add the vanilla and beat for another 30 seconds.
We will add the flour in 3 batches to the wet ingredients.
Add a third of the flour along with 1-2 tbsp milk and beat or whisk at a low speed until well combined. You can also use the cut and fold method to incorporate the flour into the mixture.
Repeat with the next batch of flour with another tablespoon of milk and then the next unit all of the dry ingredients are incorporated into the wet ingredients with the help of 1/3 cup milk in all to get a silky smooth batter that falls in ribbons when poured with a ladle.
Divide this batter into two bowls. One bowl is the vanilla batter. To prepare the chocolate batter, mix in cocoa powder in 1-2 tbsp of milk and mix this into the other bowl, stirring well with a spatula until well incorporated. Don’t over mix this.
At this point, we have two batters and a prepared baking tin with the oven preheated.
Pour ladle full of vanilla batter in the baking tin, followed by the chocolate batter and keep repeating until both batters are used up. Using a skewer, swirl the batter once right through to the bottom of the pan so that in places, both batters will intermingle to create nice marbled patterns.
Keep this to bake in the preheated oven for around 45 minutes or until a skewer in the centre comes out clean.
Allow to cool on counter for 10 minutes before unfolding. Cool on wire rack for an hour or so and then slice so as to get clean slices. You can also refrigerate overnight and then slice the cake.