Preheat oven to 350°F. Lightly spray an 8x8" baking dish with nonstick cooking spray. Beat eggs on low speed in the bowl of a stand mixer fitted with the paddle attachment until combined. Add chocolate-hazelnut spread and beat until smooth. Add flour and salt and beat, scraping down sides of bowl as needed, until combined. Pour into prepared dish; smooth surface with a spatula. Bake brownies until a toothpick inserted into the center comes out clean, 25–30 minutes. Let cool, then cut into 9 pieces and serve.