Preheat the oven to 350°F. Grease a 9x9" baking pan with baking spray and line with parchment paper, leaving an overhang over the sides to allow for easy removal of the brownies.
In a large bowl, whisk together melted butter, eggs, and vanilla extract until smooth.
Add in sugars and mix well.
Sift cocoa powder and flour into the bowl and fold with rubber spatula until well combined.
Fold in chopped chocolate and walnuts.
Add batter to prepared pan and smooth to the edges.
Bake for 25-30 minutes or until center is set. (see note)
Cool in pan for 20 minutes and then transfer to a wire rack to cool completely.
Baking time: If you prefer a more gooey, underbaked brownie, take these brownies out at 25 minutes. If you like your brownies more crispy on the edges or with a slightly cakey texture, leave them in closer to 30 minutes.