Put rack in center of the oven. Preheat oven at 325°F Put parchment paper in the bottom of a 9 inch square pan (but taking care to let exceed a strip of parchment paper on each side (2 sides) and butter the two other sides). Sorry for this sentence, I had so much trouble translating it LOL.
In a bowl, mix flour and salt. Set aside.
In another bowl, with an electric mixer, beat eggs, nutella, brown sugar and vanilla until the mixture is creamy and homogeneous, about 2 minutes. At low speed, add dry ingredients alternating with melted butter.
Spread in the pan. Cook 35 to 40 minutes or until a toothpick inserted in the center of cake comes out with lumps and not completely clean.
Let cool in the pan about 2 hours. Remove from pan and cut in squares.