Brown Butter Nutella Chocolate Chunk Cookie Bars

Preheat the oven to 350F and line an 8x8 inch square pan with parchment paper. Set aside.
In a small saucepan, melt the butter over medium heat, constantly stirring until the butter begins to foam, turns golden brown, and has a nutty aroma (about 5 to 8 minutes). Note: the butter can go very quickly from light brown to burnt, so keep a close eye on it and take it off the heat as soon as it changes colour. Let the brown butter cool slightly.
In a medium bowl, add the flour, salt and baking soda. Stir to combine and set aside.
In a large mixing bowl, combine the cooled brown butter and sugars. Beat until mixed well. Add in the egg, egg yolk and vanilla, and mix well.
Add the dry ingredients to the wet ingredients. Stir until just combined. Fold in the chocolate chunks and chopped hazelnuts until just incorporated.
Place half of the cookie dough in the pan, pressing down to create an even bottom layer. Spoon the Nutella on top of the cookie base. Take the remaining cookie dough and drop heaping tablespoons on the Nutella layer and flatten it out slightly. There should still be Nutella showing through (this creates a swirling effect on top).
Bake for 30 minutes or until the edges are golden brown and the middle is still gooey. Sprinkle with sea salt (if using), and Nutella drizzle (optional) and let it cool for at least 10 minutes. Cut into nine squares, serve and enjoy!