Grease a 9*9 or 8*8 pan and preheat the oven to 350F
Mix the butter and sugar on medium speed till well combined, mixture will be crumbly.
Add the eggs one by one, beating well after each addition on medium high speed. Batter will become far creamier and fluffier.
Now add the dry ingredients to the bowl and put the mixer on low mixing JUST until the flour is 70% combined i.e. some white streaks are ok.
Use a spatula to fold in ⅔ of the hazelnuts, most of the floury streaks should disappear
Press the dough into the pan. Tip: a little plastic wrap makes this easier, simply use the wrap to press the dough down.
Drop the nutella by small spoonfuls randomly over the dough and then use a skewer or toothpick to swirl it through.
Scatter the remaining hazelnuts over top and bake for 23-28 minutes or until the edges are puffy and a skewer inserted in the middle comes out only slightly moist.
Cool, cut, enjoy!
Video
Notes
Note: if you do not have hazelnuts available you can absolutely skip them or add in chopped peanuts or walnuts instead.