Prepare the Nutella Layer: Line an 8x8-inch pan with parchment paper. Spread the Nutella into a thin, even square that fits the pan. Freeze for 30-40 minutes or until solid.
Brown the Butter: In a small saucepan over medium heat, melt the butter. Continue cooking, stirring often, until it turns golden brown and smells nutty. Remove from heat and let it cool slightly.
Make the Cookie Dough: In a large bowl, whisk together the browned butter, brown sugar, and granulated sugar. Stir in the egg, egg yolks, and vanilla until smooth.
In a separate bowl, whisk together the flour, baking soda, and salt. Add the dry ingredients to the wet and mix until just combined. Fold in the chocolate chips.
Assemble: Press half of the cookie dough into the bottom of the lined 8x8-inch pan. Place the frozen Nutella square on top. Carefully press the remaining cookie dough over the Nutella layer, covering it completely.
Bake: Preheat oven to 350°F (175°C). Bake for 25-30 minutes or until golden brown and set.
Cool & Serve: Let the bars cool in the pan for at least 30 minutes before slicing. Sprinkle with flaky sea salt if desired. Enjoy!
