Preheat your oven to 350°F (175°C). This step is crucial for achieving that perfect brownie texture.
Line a 9x9-inch baking pan with parchment paper, allowing some overhang on the sides for easy removal.
In a mixing bowl, combine the chopped semi-sweet chocolate and melted butter. Gently melt them together using a microwave or double boiler. Stir until smooth and let cool slightly.
Add the granulated sugar, salt, and vanilla extract to the melted chocolate mixture. Whisk until well combined.
Beat the eggs in a separate bowl and then stir them into the chocolate mixture until fully incorporated. The mixture should be glossy and thick.
Sift in the dark cocoa powder and cornstarch. Gently fold until just combined, being careful not to overmix.
Fold in the chocolate chips or chunks for delightful pockets of melted chocolate.
Pour the brownie batter into the prepared pan, spreading it evenly. Bake for 20-25 minutes, until set around the edges but slightly soft in the center.
Allow the brownies to cool in the pan for about 10 minutes. Use the parchment paper to lift them out onto a wire rack to cool completely before cutting into squares.
