First grind hazelnuts, until you get a meal. Then add a couple tablespoons of flour (to absorb some of the oils), then grind again.
Into a food processor, add your ground hazelnuts, all purpose flour, powdered sugar, cocoa powder (for color), egg, unsalted butter, salt and blend until you get a crumbly texture.
Put dough on a work surface (or straight onto some plastic wrap) and gently but quickly work with your hands until you get a nice dough. Then flatten it with your hands, cover with clingfilm and let chill into the fridge for about 30 minutes or more, if needed.
Take dough out of the fridge, and roll the dough out onto a well floured surface. If it's too sticky, dust well.
Shape your cookies using your favorite cookie cutters, then arrange them on prepared baking sheets (lined with parchment paper)
Let chill into the fridge for about 10 minutes or a bit more. In the meantime, preheat the oven at 180°C / 355° F
Bake for about 14 minutes, they are ready when you can smell a delicious nutty flavor.
Let cool down completely, then fill with hazelnut spread and form a sandwich.