Preheat your oven to 180°C (356°F) and line the base of a baking tray with a sheet of parchment paper.
Make a "flax egg" by adding the ground flaxseeds to a bowl with 2.5 tsp water. Stir and allow to sit for 5 minutes.
Roughly chop the dark chocolate and hazelnuts and set them aside.
Add the hazelnut butter and coconut sugar to a large bowl and beat together using a whisk (electric if possible). Add the cacao powder, vanilla extract, baking soda, and flax egg and beat again for 30 seconds or so until the ingredients have combined.
Add the three-fourths of the chopped dark chocolate and hazelnuts to the bowl and fold them through the cookie dough using a spatula.
Separate the cookie dough into 10 parts, roll them into balls, and set them on the baking tray, leaving at least 2 inches between each cookie. Use the back of a large spoon to slightly flatten the cookies down into discs. Stud the cookies with the remaining chocolate chips.
Bake for between 10-12 minutes until the cookies have spread and slightly browned. Remove the tray from the oven and while hot, use a large round cookie-cutter and go in circular motions around each cookie to create a clean edge.
Allow the cookies to sit on the baking tray to cool for about 5 minutes. Next, carefully slide the sheet of parchment paper with the cookies off of the tray and on a cooling rack. Cool for another 15-20 minutes until they reach room temperature.
Store in an airtight container at room temperature and consume within 3-4 days.