Preheat your oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper, leaving overhang on two sides for easy removal.
In a medium bowl, whisk together the flour, cocoa powder, salt, and baking powder until no lumps remain. Set aside.
In a large mixing bowl, whisk the melted butter with both sugars until smooth and glossy, about 1 minute. The mixture should be thick and well combined.
Add the eggs one at a time, whisking well after each addition until the batter becomes thick and lighter in color. Stir in the vanilla extract.
Add the dry ingredients to the wet ingredients and gently fold with a rubber spatula until just combined. Stop mixing as soon as the flour disappears—a few small streaks are fine.
Pour the batter into the prepared pan and spread it evenly to the corners. Tap the pan on the counter twice to release any air bubbles.
Bake for 28-32 minutes, until the edges look set and pull away slightly from the pan, and a toothpick inserted 2 inches from the edge comes out with moist crumbs.
Let the brownies cool completely in the pan on a wire rack, at least 2 hours. For the cleanest cuts, refrigerate for 1 hour before slicing into 12 or 16 squares.
