Preheat your oven to 180°C (350°F).
Mash the bananas in a large mixing bowl.
Add the vegetable oil, vanilla extract, egg (or flax egg), yogurt, caster sugar, and light brown sugar. Mix until combined and smooth.
Fold in the self-raising flour, ground almonds, baking powder, and cinnamon. Stir until you have a smooth batter.
Divide the batter equally into two bowls. Add the cacao powder to one bowl and mix until smooth.
In a greased loaf pan, alternate spoonfuls of plain and chocolate batter until you have used all the batter.
Use a knife or skewer to swirl the batter, creating a marbled effect.
Bake for 45-55 minutes or until golden brown and a skewer inserted into the center comes out clean.
Let cool before slicing and serving.