Preheat the oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal. Lightly grease the parchment paper with butter or cooking spray.

In a large mixing bowl, whisk together the melted butter and granulated sugar until glossy and smooth. Add the eggs one at a time, whisking thoroughly after each addition. Stir in the vanilla extract until combined.

In a separate bowl, sift together the all-purpose flour, unsweetened cocoa powder, and salt. Ensure the mixture is evenly combined and free of lumps.

Gradually fold the sifted dry ingredients into the wet mixture using a spatula. Mix gently until no streaks of flour remain, taking care not to overmix to maintain a thick, fudgy batter.

Fold in the semi-sweet chocolate chips or chunks to evenly distribute them throughout the batter.

Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 30–35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached.

Allow the brownies to cool in the pan for at least 30 minutes. Use the parchment overhang to lift the brownies out of the pan and transfer them to a cutting board. Slice into squares or rectangles as desired.