Preheat the oven to 350 degrees F and line a 9 inch square pan with parchment paper.
In a small bowl, add the chocolate and butter. Place over a small saucepan of simmering water, and stir often until fully melted. Remove from the double boiler and allow to cool for a few minutes.
Meanwhile, in a large bowl, add the sugars, eggs, vanilla, and salt. Using an electric mixer, beat on high speed for about 3-4 minutes until the mixture has doubled in size.
While mixing on low speed, slowly pour in the chocolate butter mixture.
Once it is fully combined, add the flour, cocoa powder, and chocolate, and mix on low speed until fully combined.
Set aside as you make the blondie layer.
For the blondie layer:
In a medium bowl, whisk together the melted butter and brown sugar until fully combined.
Then, mix in the egg, vanilla and salt.
Fold in the flour and baking powder until just combined.
Lastly, fold in the chocolate.
For assembly:
To the prepared square pan, alternate the brownie and blondie batters and smooth out the top, careful not to swirl them too much.
Lastly, top the brownie blondies with more chopped chocolate, if desired.
Bake for about 28-32 minutes or until the blondie layer is golden and the brownie layer appears set.
Allow the brownie blondies to cool completely in the pan before removing.
Cut into diamonds or squares, and enjoy!
Notes
Use a high-quality bittersweet chocolate. I recommend Valrhona or Guittard.
Beat the sugars and eggs for 3-4 min. Be sure to beat the sugars and eggs for a full 3-4 minutes! This is what will give the brownie sections that gorgeous shiny top.
How to cut brownie blondies: To get clean cut squares, use a sharp knife and run it under hot water (and wipe clean) before each cut.