Preheat oven to 350F; line a 9 inch square baking pan with parchment paper; lightly grease parchment paper
In a microwave safe bowl, melt chocolate and butter until smooth; allow to slightly cool
In a large bowl, whisk together chocolate mixture, sugar and salt; whisk in eggs one at a time; add vanilla extract; sift in flour and cocoa powder; stir until batter comes together
Pour batter into prepared pan
Drop Nutella in dollops around the batter; use a butter knife to swirl the Nutella through the batter
Bake for 35 minutes until toothpick comes out with a few moist crumbs; allow to cool completely before slicing
