Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper so that the paper overhangs by a few inches on two opposite sides.
Place the chocolate and butter in a microwave-safe bowl. Heat in the microwave at half power in 30-second increments until the butter is melted and the chocolate melts when stirred. Stir until smooth. Set aside to cool slightly.
Mix the sugar, brown sugar, eggs, and vanilla until well-blended. Add the chocolate-butter mixture, and mix until smooth.
Add the flour, cocoa powder, and salt, and mix just until combined.
Transfer the batter to the prepared pan and spread evenly. Bake 35-40 minutes, or until a pick inserted into the center comes out with a few moist crumbs attached.
Top with favorite frosting,