In a food processor, grind the raw hazelnuts until they reach a coarse flour consistency.
In the food processor, combine all the ingredients except for the hazelnut flour and chocolate chips. Process until well mixed into a thick paste (I used an immersion blender).
Add the hazelnut flour back into the food processor and pulse briefly to combine, or, if you use an immersion blender, mix it into the paste with a spatula.
Transfer the mixture to a bowl and stir in most of the chocolate chips, reserving a few for garnish.
Pour the batter into the greased baking dish, smoothing the top with a spatula. Sprinkle the reserved chocolate chips over the top, and some sea salt flakes.
Bake for about 25 minutes in a preheated oven at 350F / 175C. The top may still be soft but it will firm up as it cools.
Allow the brownies to cool completely on a cooling rack. For best results, refrigerate them for about an hour before serving to allow them to fully set.