Preheat the oven to 175C | Gas mark 3.
Line a loaf pan with baking parchment and set aside.
In a large mixing bowl, mash the bananas using a fork. Add the light brown sugar, vegetable oil, vanilla extract, and soya milk then stir to combine.
Sift in the plain flour, baking soda, salt, and cinnamon. Mix to incorporate.
Transfer half of the batter into a bowl. Add the cocoa powder and mix to combine and until smooth.
Spoon both mixtures alternating as you go into the prepared loaf cake tin.
Drag a knife through the mixture a few times to marble the cake.
Bake for about 45 minutes or until a toothpick inserted in the centre comes out clean.
