With the rack in the middle position, preheat oven to 325 degrees. LIne the bottom of a 20 cm (8 inch) square cake pan with parchment paper, letting the paper extend over two opposite sides. Butter the two other sides.

In a bowl, combine the flour and salt. Set aside.

In another bowl, beat the eggs, Nutella, brown sugar and vanilla extract with an electric mixture until smooth, about 2 minutes. With the mixer on low speed, add the flour mixture, alternating with the melted butter.

Scrape the batter into the cake pan. Bake until a toothpick inserted in the centre comes out with a few crumbs attached (not completely clean), 35 to 40 minutes. Cool in the pan for about 2 hours. Unmould and cut into squares.

Serve warm or at room temperature.