Preheat the oven to 325˚F. Line an 8×8 baking pan with parchment paper.
In a medium bowl, mix together the salt and flour.
In a separate bowl, beat together the Nutella, brown sugar, vanilla and eggs until smooth.
Alternate between adding the flour and the butter into the Nutella mixture, and blend until fully combined.
Pour batter into the prepared pan. Drop tablespoons of the dulce de leche over the brownies. Using a knife, carefully swirl the dulce de leche into the brownie mixture.
Bake in the preheated oven for 50-55 minutes, or until a toothpick inserted comes out with only a couple of crumbs.
Remove from oven, and let cool for about an hour.