Preheat the oven to 180C | Gas Mark 4. Line a loaf pan with baking parchment and set aside.
In a large mixing bowl, combine both the flours, sugar, baking powder and salt. Make a well in the middle of the dry ingredients and add the milk, oil and vanilla extract. Stir until combined.
Transfer half of the mixture into a separate bowl. Add the cacao powder and more soya milk, then stir to incorporate.
Using a separate spoon for each batter, spoon the batter into the tin. Begin from the middle of the tin, and alternate between the white and chocolate batter.
Once all the batter is used up, simply use a bamboo skewer or a knife to create a marble pattern by dragging it through the mixture in swirls.
Bake in the middle of the oven for 50 minutes or until a skewer inserted in the middle of the cake comes out clean. Transfer onto a wire rack and let cool completely before serving.