To a bowl, add the plain flour, salt and bicarbonate soda then whisk them together and set aside. To a stand mixer bowl, add the butter and both sugars. Using a paddle attachment on a medium setting, cream them together for around 3 minutes or until light and fluffy. You can also use a hand mixer for this step.
Add in the banana and continue to mix for another minute. Mix in the vanilla extract then pour in the dry ingredients that you set aside earlier and continue to mix until a dough forms. Once a dough forms, add in the hazelnuts and combine well.
Cover the bowl and place the dough in the fridge for 20 minutes. After 20 minutes, line a large plate with parchment paper then roll the dough into 16 even sized balls. Each ball should weigh between 36g and 37g. Place the dough balls on the plate as you roll them. Refrigerate the cookie dough balls for 2 hours.
Preheat the oven to gas mark 4/350oF/180oC/160oC fan and line a baking tray or cookie sheet with parchment paper. Place the cookies on the tray ensuring you leave space between them to spread. Add some chopped hazelnuts onto the top of each cookie then bake in the oven for 14 minutes or until lightly browned. Allow the cookies to cool on the baking sheet for 25 minutes then transfer them to a cooling rack to completely cool.