Preheat the oven to 180oC/Gas Mark 4.
Peel and chop the sweet potato into small cubes. Fill a pan with water and bring it to a boil. Cook the sweet potato for about 10 minutes until completely soft, then drain and set aside.
Add the butter to a pan on medium heat and melt. Remove it from the heat and add the banana, sweet potato, agave nectar and cinnamon. Using a fork or masher, mash everything together until smooth.
Add the oats and chocolate chunks and stir until well mixed, then transfer the flapjack mixture onto a baking tray. Bake for about 15 minutes until golden, then remove from the oven and set aside to cool completely before slicing and serving.