Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving some overhang on the sides for easy brownie removal.
In a medium saucepan, combine the chopped semi-sweet chocolate and melted butter over low heat. Stir gently until completely melted and smooth. Remove from heat and let it cool slightly.
Once the chocolate mixture has cooled a bit, whisk in the granulated sugar, salt, and vanilla extract until combined.
Beat the eggs in a separate bowl, then add them to the chocolate mixture. Stir until the mixture is glossy and smooth.
Sift in the dark cocoa powder and cornstarch. Gently fold the dry ingredients into the chocolate mixture using a spatula until just combined. Do not overmix.
Fold in the chocolate chips or chunks for added texture and richness.
Pour the brownie batter into the prepared baking pan and spread evenly. Bake for 20-25 minutes, until edges are set and center has a slight jiggle. Toothpick should come out with moist crumbs.
Remove from oven and let cool in the pan for 15 minutes. Lift brownies out using parchment paper and cool completely on a wire rack. Cut into squares and serve.