Preheat your oven to 175°C (350°F). Grease a 9x9-inch baking pan and line with parchment paper, leaving an overhang on two sides for easy removal.
In a heatproof bowl over a saucepan of barely simmering water, melt the chopped dark chocolate and butter together, stirring until smooth. Remove from heat and let cool for 5 minutes. You can also do this in the microwave in 30-second bursts.
Whisk the granulated sugar and brown sugar into the chocolate mixture until fully combined.
Add the eggs one at a time, whisking vigorously after each addition. Stir in the vanilla extract. The batter should look glossy and slightly thickened.
Sift the flour, cocoa powder, and salt directly into the bowl. Use a rubber spatula to fold until just combined — stop as soon as no dry streaks remain. Do not overmix. Fold in the chocolate chips if using.
Pour the batter into the prepared pan and smooth the top. Bake for 28–32 minutes, until the edges are set and a toothpick inserted 2 inches from the edge comes out with moist crumbs (not wet batter). The center should still look slightly underdone.
Let the brownies cool in the pan on a wire rack for at least 1 hour before lifting out and slicing. For the cleanest cuts, refrigerate for 30 minutes before slicing with a sharp knife, wiping the blade between cuts.