Preheat the oven to 350 degrees fahrenheit

Grease and line a 9x13” metal pan with parchment paper. Set aside.

In a small saucepan, melt the butter over low heat. This step can also be done in the microwave.

Remove from the heat and add the nutella and cocoa powder.

Set chocolate mixture aside to cool.

In a medium mixing bowl, combine the eggs, sugars, salt, and vanilla. Using a mixer fitted with the whisk attachment (or hand mixer), beat on high speed until the mixture thickens and becomes pale in color and falls from the beater in a wide ribbon that folds back on itself and slowly dissolves on the surface, 4 to 5 minutes.

Using a rubber spatula, fold the cooled chocolate into the egg mixture.

Add the flour and fold it in quickly but gently with the rubber spatula so that you don’t deflate the air that’s been incorporated into the eggs.

Pour the batter into the prepared dish and smooth the top with a spatula.

Bake until the top looks slightly cracked and feels soft to the touch, about 30 minutes.

Let cool completely on a wire rack.

Using a sharp knife, cut into 12 squares, or size desired.