Preheat your oven to 160°C and grease and/or line a cake tin with parchment paper.
In a large bowl, mash the bananas.
Add the yogurt, eggs, vanilla extract and 3 tbsp maple syrup.
Then in a separate bowl, mix the whole wheat flour, oat flour, baking powder and baking soda. Add the dry mixture into the wet and mix until just combined.
Pour about 2/3 of the mixture into the prepared cake tin.
Then add the cocoa powder, milk and 1 tbsp maple syrup to the remaining batter and mix well.
Pour into the cake tin.
Now using a spoon or a knife, swirl through the batter to create a marble pattern.
Bake for about 40-45 minutes until a tooth pick inserted in the middle comes out clean (a few crumbs is fine).