Preheat the oven to 325 degrees Fahrenheit with a rack in the lower third of the oven.
Line an 8-inch square baking pan with parchment paper to lift out easily when brownies are cooled and spray with non-stick spray. (parchment paper should overhang a couple inches.)
Melt the butter in a medium-sized saucepan over medium heat, whisking constantly, until it’s a pale golden brown and the particles suspended in it are reddish brown for approx 10 minutes to make browned butter.
Remove pan from heat and then stir in the sugar.
Add the cocoa powder, sea salt and baking powder and stir until the ingredients are thoroughly incorporated. Let the mixture cool for 5 minutes.
Add the eggs one at a time whisking until incorporated well.
When the mixture looks thick and shiny, add the vanilla extract and the flour.
Stir until you no longer see streaks of flour, then beat vigorously for 50 strokes with the wooden spoon or spatula.
At this point, the mixture should be no more than slightly warm (if not, let it cool for a few more minutes). Fold in the chocolate chunks or chips.
Spread the batter in the lined pan, then use a knife to make light swirls in the top of the batter. Bake on the lower rack for about 25 minutes, until a toothpick inserted in the center comes out almost clean. Cool the brownies in the pan on a baking rack.
Once the brownies are completely cool, lift the edges of the parchment paper and transfer the brownies to a cutting board.