Mash the bananas with a fork in a mixing bowl. Add the oats, ginger and chopped hazelnuts, mix well, and leave to rest for 10 minutes.
Break the chocolate bar into small pieces and place them in a small saucepan. Add the oat milk and heat gently until the chocolate has just melted and formed a thick, smooth cream. Remove from the heat and allow to cool slightly.
Preheat the oven to 180°C and line a baking tray with parchment paper. Alternatively, preheat your air fryer.
Add the cooled chocolate mixture to the oat batter and mix well. If the mixture is too runny, add a little more oats. If too thick, add a splash of oat milk. It should be thick but malleable.
Shape the mixture into evenly sized balls and gently flatten them between your palms. Place the cookies on the prepared tray.
Bake in the middle of the oven for 10–15 minutes, until lightly golden. Alternatively, bake in the air fryer for about 12 minutes, turning them once halfway through.
Allow the cookies to cool on a wire-rack before serving.