01 - Preheat the oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - Whisk together flour, cocoa powder, and salt in a medium bowl until evenly mixed.
03 - In a large bowl, whisk melted butter with granulated and brown sugars until smooth and glossy.
04 - Add eggs, egg yolk, and vanilla extract to the butter-sugar mixture. Whisk thoroughly for about 1 minute until combined.
05 - Gently melt chopped chocolate over a double boiler or in 20-second microwave intervals, stirring until smooth. Allow to cool slightly before adding.
06 - Whisk the melted chocolate into the wet ingredients until evenly incorporated.
07 - Add dry ingredients to the wet mixture and fold gently using a spatula until just combined; avoid overmixing.
08 - Fold in chocolate chips or nuts if using, ensuring even distribution.
09 - Pour batter into the prepared pan and smooth the surface with a spatula.
10 - Bake for 28 to 32 minutes or until a toothpick inserted in the center comes out with a few moist crumbs but no wet batter.
11 - Allow brownies to cool completely in the pan on a wire rack before lifting out and cutting into 16 squares.