Preheat Oven to 350 degrees Fahrenheit
Line an 8x8 square pan with parchment paper. Leave paper overlapping on the sides for easy removal. Spray with non-stick spray
In a pan over medium-low heat, melt the butter and chocolate chunks together. Let them cool for 2-3 minutes
Meanwhile, in a bowl, whisk the eggs, sugar, flour, and salt. Add the melted chocolate mixture, the Nutella, hazelnuts, and extract. Mix until combined
Pour mixture in the baking pan and smooth out the top with a spatula or spoon
Place in the oven for 20-24 minutes. Mixture will be cracked a bit on top but still soft inside
Let it cool on a baking rack and chill in the fridge for a couple of hours
After it's cooled, lift the sides of the parchment paper and cut into pieces
Enjoy!