Place the chocolate in a microwave-safe bowl. Heat in the microwave at half power in 30-second increments until the chocolate melts when stirred. Set aside to cool slightly.

Using an electric mixer on medium speed, beat the butter, sugar, and brown sugar until light and fluffy. Add the eggs and vanilla, and mix until well blended.

Add the melted chocolate to the batter. Beat on medium speed until well blended and smooth.

Reduce mixer speed to low. Add about 1/3 of the flour and all of the salt. Mix just until combined. Add the remaining flour in two portions, mixing just until combined. Stir in the chocolate chunks.

Transfer the batter to the prepared pan. Use an offset spatula to spread evenly and smooth the top.

Bake 25 to 30 minutes, or until a pick inserted into the center comes out clean.

Place the pan on a wire rack and allow the cake to cool completely.

Remove the brownies from the pan. Dust the top of the cake with cocoa powder before serving.