Preheat the oven to 350°F (175°C). Line the baking pan with parchment paper for easy removal. Set aside.
In a large mixing bowl, combine melted butter and sugar. Whisk until thoroughly blended and slightly glossy, indicating sugar begins to dissolve.
Add eggs one at a time, whisking well after each addition. Observe the mixture becoming lighter in color and slightly airy as eggs incorporate.
Stir in sifted cocoa powder and vanilla extract until fully integrated. The mixture will darken and thicken slightly, releasing a rich chocolate aroma.
In a separate bowl, whisk together flour, baking powder, and salt. Gradually add dry ingredients to the wet mixture, folding until just combined. The batter should be thick and slightly fudgy.
Gently fold in chocolate chips. Pour the batter into the prepared pan and spread evenly with a spatula. Place in the oven and bake for about 25 minutes, until the edges are visibly darker and just starting to burn slightly, and a toothpick inserted into the center comes out with moist crumbs.
Remove from oven and allow to cool in the pan for 10 minutes before transferring to a wire rack. The edges will be dark and caramelized, with a shiny, crackled surface, and the center soft and chewy.