Preheat the oven to 350 F/180 C. Place a sheet of parchment paper inside of a square 8x8 baking pan, creasing the bottom edges well so that when you remove it from the pan it has an outline of the size of the pan.
Spread the Nutella over the parchment within the creased lines. Chill the Nutella layer in the freezer, it must be firm before using.
Line the baking pan with parchment paper on all four sides, then set it aside.
Brown the butter in a stainless steel pan over medium heat until the bottom is covered in brown butter solids. Scrape the butter into a bowl and cool to room temperature. You can chill it in the fridge or freezer, stirring every 5 minutes, until it's ready. Just keep an eye on it!
In a large bowl, whisk together the wet ingredients. Then stream the butter into the bowl, whisking until combined.
Fold in the dry ingredients. Spread half (about 450 g) of the cookie dough into the pan. Then place the slab of Nutella on top of the dough. Spread the rest of the cookie dough over the top.
Spoon dollops of the warmed Nutella on top of the cookie dough and swirl it with a toothpick. Bake the Nutella bars for 30-40 minutes, or until a toothpick inserted into the center comes out covered in a few moist crumbs.
Let the cookie bars cool completely on a wire rack, then cut into 16 bars. Enjoy!
Store leftovers in an airtight container at room temperature for 3-4 days. You can also freeze them in an airtight container or freezer bag for up to 1 month.