Preheat the oven to 350°F. Spray a 9×13-inch baking pan with pan spray and line the pan with parchment paper, leaving an overhang on two opposite sides; set the prepared pan aside.

In a medium bowl, whisk together 1 cup melted unsalted butter and 2 1/4 cups granulated sugar until well combined. Let the mixture cool for a minute or two if the butter is very hot.

Add the 4 large eggs, one at a time, whisking until each is incorporated. Whisk in 1 tablespoon vanilla until smooth.

In a separate bowl, sift (or whisk) together 1 1/3 cups all-purpose flour, 1 teaspoon salt, 1 teaspoon baking powder, 3/4 cup cocoa powder, and 1/4 cup dark cocoa powder until evenly combined.

Add the dry ingredients to the wet ingredients in two additions. Fold gently with a spatula until just combined—do not overmix.

Stir in 4 ounces chocolate chips or chunks until evenly distributed through the batter.

Pour the batter into the prepared 9×13 pan and spread it evenly with a spatula.

Bake at 350°F for 30 to 40 minutes. For fudgier brownies, bake 30–35 minutes; for firmer, more set brownies, bake 35–40 minutes. Test doneness by inserting a toothpick into the center: fudgy brownies will remove with a few moist crumbs, while more set brownies will come out mostly clean.

Remove the pan from the oven and place it on a wire rack. Cool the brownies in the pan for at least 30 minutes.

Use the parchment overhang to lift the brownies from the pan, place on a cutting board, slice into squares, and serve.