Preheat the oven to 160°C/320F and line a baking tray with greaseproof paper.
Make the flax eggs: add the ground flax seeds and water to a dish and stir. Let it rest for 5 minutes to thicken.
Take 1 cup (150 g) of hazelnuts and grind them into a coarse powder in a food processor. Don't worry if you get some bigger chunks. They will just add extra texture to the cookies. Set aside the ground hazelnuts in a mixing bowl.
In the same food processor, grind the Medjool dates into a paste, by adding 1 tablespoon of water and the flax egg.
Chop the remaining 1/3 cup (50 g) of hazelnuts in chunkier pieces and spread them onto a tray.
In a big mixing bowl, combine the ground hazelnuts, date paste, salt, and cacao powder. Mix it all well to bind all the ingredients together and produce a thick, sticky paste that can be formed into sticky balls in your hands.
Divide the dough into 10 equally sized pieces. Roll each portion of dough between the palms of your hands to form a ball. Squash the ball a bit to give it a cookie shape then lightly press each one into the chopped hazelnuts to coat.
Place the cookies on a parchment-lined baking tray. Bake for about 10 minutes.
Remove from the oven and let the cookies cool down before moving them to a plate or airtight container. Be careful as they are quite crumbly when they are hot. Resist the urge to try one warm, as they are much better when they cool down.