Preheat the oven to 350 degrees F (180 degrees C). Grease a 9x13 pan and line with parchment.
Melt Butter (3/4 cup) in a small saucepan. Continue to cook over medium heat until the butter foams and starts to brown.
Take off the heat and add in the Brown Sugar (2 cup). Stir until combined, then let the butter/sugar mixture cool for about 10 minutes.
Add in the Egg (2) and Vanilla Extract (2 teaspoon).
In a separate bowl, whisk together the All-Purpose Flour (2 cup), Baking Powder (1 teaspoon), and Baking Soda (1/4 teaspoon). Add the dry ingredients to the wet ingredients and stir together until well combined.
Scoop out half of the dough all around the pan and smooth out as best as possible. Scoop out the Nutella® (1 cup) and smooth across the dough evenly. Repeat with the top layer of batter. Sprinkle the Chocolate (1 bar) on top. Place in the oven for 20-25 minutes.
Take out of the oven and sprinkle with Sea Salt Flakes (to taste).
Wait at least 30 minutes before cutting and serving.