Make the oat flour by grinding half of the oats. You can do this is a spice grinder or a blender/food processor.
Chop the hazelnuts - some smaller and larger bits are fine. You could also blitz them in a blender for a second, to crush them ( especially if you want smaller pieces).
If you don't have chocolate chips and only a bar of chocolate, then chop that too, into smaller pieces.
In a large bowl, mix the dry ingredients - rolled oat, oat flour, hazelnuts, chocolate, cinnamon and salt.
Mix the wet ingredients in a separate bowl - mash the banana and add the coconut oil and maple syrup.
Mix the dry and wet ingredients together. Make sure to incorporate well. If the mixture seems too dry, add a bit more maple syrup or coconut oil. If you are wanting to add any additional ingredients then you can add them now too.
Make small balls from the mixture. You can use a measuring spoon to scoop out the same amount of the mixture.
Place the balls on a baking sheet with parchment paper and slightly flatten the balls using your hands or a small cup. These cookies won't spread in the oven at all so you can fit them as close to each other as you like (see how I do it in the video)
Bake in the oven at 170ºC (fan-assisted) for 10-12 minutes. If you like the cookies to be more crunchy and golden, leave them a couple of extra minutes in the oven but keep an eye on them so they don't burn.
Let them cool to room temperature before removing from the baking tray and transferring to an airtight container. These can be kept for 2-3 days in the fridge or frozen for up to 3 months.