I make these in a stand mixer with the paddle attachment-- just a heads up.
Add the butter and sugars to the bowl and mix for a couple minutes on medium until creamy and fluffy.
Scrape the bowl. Add eggs one at a time, then the vanilla and mix for about 30 seconds between each.
Scrape the bowl. Add the salt and baking powder, mix for another minute or so.
Scrape the bowl again-- add 1 cup of flour and mix on low until mostly incorporated. Then scrape the bowl and repeat with the second and third cups. I usually end up adding about 1/4 cup more flour because I use extra large eggs. If you're using eggs you'll probably only need 3 cups.
I mince my walnuts in a food processor, but you can totally chop them if you're into that. The reason I mince them is because I don't actually like nuts in my cookies. But the flavor is awesome and they keep the cookies very moist. So there ;)
Add the nuts and chocolate chips to the bowl and mix on low until mixed, about 30 seconds. Then dump onto a piece of plastic wrap. wrap it up tight and put it in the fridge for a little bit to set up. I usually leave it in there for about 30 minutes before baking.
Using a measuring spoon, get 1 tsp chunks of dough, roll them into balls, and then flatten them a little on a piece of parchment paper. I can usually get about 20 on a 13 x 9 pan but the recipe makes lots of dough. Bake on 350 for about 8 minutes- any longer and you'll get crunchy cookies.
Remove the parchment paper from the pan and allow the cookies to cool completely. And I mean completely, or else the Nutella will go everywhere. Then put a little blob of Nutella on half of the cookies and sandwich them together. Boom! Now eat them, hurry before they disappear!