Beat the butter and brown sugar until you have a creamy texture. You can do it by hand using a whisk or with an electric mixer.
Add the egg and vanilla essence and beat again until creamy.
Add the salt, flour, oat flour, baking powder and rolled oats and mix. Finish by adding the chocolate chunks and mixing them in.
Form balls of 2 tbsp worth of dough between your hands. Flatten them slightly and place them on a baking tray with silpat or baking paper. Leave about 3 fingers between the cookies as they grow in the oven.
Bake in a preheated oven at 180°C / 350°F for 10-15min or until golden underneath. When you take them out of the oven they will be soft! Wait for them to cool down slightly before removing them from the tray. If they're hard then you take them out, it means you've cooked them for too long.
Optional: top the cookies with extra chocolate and coarse sea salt when they come out of the oven.