Preheat your oven to 175°C (350°F). Line an 8x8-inch baking pan with parchment paper, leaving overhang on the sides.
In a large microwave-safe bowl, combine the butter and chopped dark chocolate. Microwave in 30-second intervals, stirring between each, until fully melted and smooth. Let cool for 5 minutes.
Whisk the granulated sugar into the melted chocolate mixture. Add the eggs one at a time, whisking well after each addition. Stir in the vanilla extract. The batter should look glossy.
Sift in the flour, cocoa powder, and salt. Fold gently with a spatula until just combined — do not overmix. Fold in the chocolate chips if using.
Pour the batter into the prepared pan and spread evenly. Bake for 22–25 minutes, until the edges are set and a toothpick inserted in the center comes out with a few moist crumbs.
Let the brownies cool completely in the pan before lifting out and slicing into 16 squares. For clean cuts, refrigerate for 30 minutes before slicing.