Line an 8-inch square baking pan with a piece of parchment paper so that it hangs over two sides — this will make it easier to remove the brownies.
Whisk the eggs in a mixing bowl with the vanilla and chocolate-hazelnut spread until smooth. Stir in the sugar, baking soda, salt and half of the chopped chocolate. Continue stirring until blended.
Scrape the batter into the prepared pan, smoothing it into the corners. Sprinkle the remaining chocolate and the hazelnuts over the top.
Bake 30 minutes. Remove from the oven and sprinkle the flaky sea salt over the top.
Let cool in the pan completely. Carefully lift the brownies out of the pan and cut into squares (you can chill the baked brownie in the refrigerator for 20 minutes before slicing if you find them too gooey).
Notes
I love Nocciolataspread for this recipe— it's a little harder to find than the plastic supermarket tubs of Nutella, but it doesn't include palm oil, and it's not as sugary sweet.