Preheat the oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
Melt the butter and chopped dark chocolate together in a microwave-safe bowl or over a double boiler, stirring until smooth. Let cool for 5 minutes.
Whisk in the granulated sugar, brown sugar, eggs, and vanilla extract until glossy and well combined.
Add the flour, cocoa powder, and salt. Fold gently until just combined; do not overmix.
Stir in the chocolate chips, if using.
Spread the batter evenly into the prepared pan.
Bake for 25 to 30 minutes, or until the edges are set and a toothpick inserted in the center comes out with a few moist crumbs.
Cool completely in the pan before lifting out and slicing into 12 brownies.