Preheat the oven to 325 degrees and position a rack in the upper third of the oven. Line an 8-inch square baking pan with parchment paper, leaving an overhang on each side.

Melt the butter in a medium saucepan set over low heat. Add the granulated sugar, the dark brown sugar, cocoa powder, and salt. Mix well. Cool slightly. (The mixture should be lukewarm.)

Vigorously stir in the vanilla and eggs. When the batter is shiny and well combined, add the flour and stir until just combined. Stir in the chopped chocolate, and spoon the batter into the prepared baking dish. Bake until a toothpick inserted in the center comes out with a few moist crumbs clinging to it, about 40 minutes. Using the parchment overhang, transfer the brownies to a cooling rack to cool completely. Cut into 16 squares and serve.