Preheat your oven to 350°F (175°C) to ensure even baking.
Line your 9x9-inch baking pan with parchment paper, leaving some overhang on the sides.
In a medium saucepan over low heat, combine the chopped semi-sweet chocolate and melted butter. Stir until fully melted and smooth. Remove from heat and let cool slightly.
In a large mixing bowl, combine the granulated sugar, salt, and vanilla with the melted chocolate mixture. Stir until well incorporated.
Slowly add the beaten eggs to the mixture, stirring continuously to ensure they are well combined without cooking the eggs.
Sift the dark cocoa powder and cornstarch into the bowl. Gently fold them into the brownie mixture until just combined.
Fold in the chocolate chips or chunks for an extra gooey texture.
Pour the brownie batter into the prepared baking pan, using a spatula to spread it evenly.
Bake for 20-25 minutes. The brownies are done when a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for about 10 minutes before lifting them out using the parchment overhang. Let them cool completely on a wire rack before cutting into squares.
