Preheat the oven to 325F with a rack in the lower third of the oven. Line an 8-inch square baking pan with two criss-crossed pieces of parchment paper, making sure that the paper is long enough to go up the sides a couple of inches. Grease the parchment paper.

Brown the butter: Melt the butter in a medium-sized saucepan over medium heat. Continue to cook the butter, while whisking constantly, until it’s a pale golden brown and the particles suspended in it are reddish brown. This usually takes me about 10 minutes.

Remove the pan from heat and stir in the sugar. Then add the cocoa powder, sea salt, baking powder and espresso powder. Stir until the ingredients are thoroughly incorporated. Let the mixture cool for 5 minutes.

Add the eggs one at a time, beating vigorously with a wooden spoon or silicone spatula after each one. When the mixture looks thick, shiny and well blended, add the vanilla extract and the flour. Stir until you no longer see streaks of flour. Then beat vigorously (put those arm muscles to work!) 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 mostly 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. Use a sharp chef’s knife to cut the brownies into 16 or 25 squares.