Preheat the oven to 350 degrees and line an 8 x 8 pan with parchment paper leaving an overhang on 2 sides or spray the pan with cooking spray.
Combine butter and chopped chocolate (or chocolate chips) in a medium heatproof bowl. Microwave in 30 second increments, stirring each time, until completely melted.
Add the brown and white sugar and stir to combine.
Add the eggs and stir until combined.
Add the cocoa powder, flour and salt and stir until almost combined but streaks of flour are still visible. Stir in the chocolate chips/chunks until just combined.
Pour the batter into the prepared pan, smoothing the top.
Bake for about 30 minutes, until the edges are slightly puffed and the middle is set. The toothpick test won’t work for these because of the fudgy texture. You can also use an instant-read thermometer to test the middle of the brownies. You would want a temp around 180-190 degrees Fahrenheit.
Let the brownies cool slightly and then refrigerate them for a couple of hours to allow them to solidify and make them easier to cut.