Preheat the oven to 350°F. Liberally spray a 9×9-inch metal nonstick baking pan with cooking spray. Line with parchment paper leaving a 2-inch overhang. Spray the parchment paper with cooking spray. Set aside.
Melt the butter and unsweetened chocolate in a medium saucepan over medium heat while stirring. Set aside to cool slightly.
To the pot add the vanilla extract, granulated sugar and brown sugar. Beat by hand with a hand mixer on low speed or a large spoon, until fully combined and the sugar begins to melt. (It won't melt fully.)
Add the eggs and egg yolk beating by hand until glossy.
In a separate small bowl, use a whisk to sift together all purpose flour, cocoa powder and salt.
Add sifted flour mixture beating until fully distributed and a batter is formed. Do not over mix. Fold in the chocolate chips by hand.
Spread the batter evenly into the prepared pan. Sprinkle top of the brownies with additional chocolate chips, if desired.
Bake for 25-35 minutes until a toothpick inserted into the center shows moist crumbs.
Cool in the pan on a wire rack.
Remove the brownies from the pan using the parchment paper. Slice and serve.