01 - Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a medium bowl, whisk together flour, cocoa powder, and salt 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 until fully combined, about one minute.
05 - Gently melt the chopped chocolate using a double boiler or microwave in 20-second intervals, stirring until smooth. Allow to cool slightly, then whisk into the wet mixture.
06 - Fold dry ingredients into the wet mixture gently with a spatula until just blended; avoid overmixing.
07 - If desired, fold in chocolate chips or chopped nuts evenly.
08 - Pour the batter into the prepared pan and smooth the surface evenly.
09 - Bake for 28 to 32 minutes, or until a toothpick inserted into the center comes out with moist crumbs but no wet batter.
10 - Let brownies cool completely in the pan on a wire rack before lifting out and slicing into squares.