Preheat the oven to 180°C (gas mark 4.5) and grease a 8 x 4 inch loaf tin, lightly dust with flour.
Sift the dry ingredients together in a large mixing bowl. Mix all of the wet ingredients in a separate bowl.
Add the wet mixture to the dry and whisk until just combined without any lumps remained.
Pour about 1/2 amount of the batter into another bowl and mix in 1 tbsp cocoa powder and 2 tbsp of soya milk to make the chocolate batter.
You want to make the chocolate batter the same consistency as the vanilla batter. If the chocolate batter looks thicker, add a little more milk to adjust.
Now you have a bowl of the vanilla batter and a bowl of the chocolate batter. Pour 3-4 table spoons of the vanilla batter into the tin and spread it evenly to make a thin layer.
Then put 3-4 table spoons of the chocolate batter on top of the vanilla layer in the tin, gently spread over. careful not to press to hard, you don’t want to mix them yet!
Repeat the steps to make layers of the vanilla and the chocolate until no more batter is left.
Using a knife or a chopstick, swirl around the batter once. You could do twice but the batter could get mixed too much and might not show clear swirl pattern.
Sprinkle some chocolate chips (optional) on top and bake for 60 to 70 minutes until a toothpick inserted in the middle of the came comes out clean. Allow it to cool completely before serving.