Preheat the oven to 180 º C.
To make the hazelnut flour, blitz toasted hazelnuts in the food processor until you have a powder.
In a bowl, mix the flours, baking powder and baking soda. Set aside.
Beat the softened butter with sugars until creamy and paler in color.
Add the egg and vanilla. Beat for a few more minutes.
Add the dry ingredients and keep beating, just enough to incorporate them in the mix.
Set aside 50 g of chocolate chips for the decoration. Stir the remaining chocolate into the cookie dough.
Spread roughly half of the cookie dough on a deep pie dish of about 24 cm in diameter.
Warm up the nutella for a few seconds in the microwave, so it is easier to spread. Distribute it on top of the cookie, leaving a border of about 2 cm.
Spread the remaining cookie dough on top of the nutella, making sure to "close" the borders of the cookie.
Decorate the top of the cookie with the remaining chocolate chips and coarsely chopped hazelnuts.
Bake for about 30 to 35 minutes, or until the cookie is set and golden. Test with a skewer - it should come out dry in the edges but still moist in the center, for a fudgy cookie. For a dryer cookie, bake for an extra 5 minutes.
Serve still warm or reheated in the microwave, to enjoy with a scoop of your favorite ice cream.