Adjust the oven rack to the lowest position and preheat oven to 350°F (175°C). Line a 9×13-inch baking pan with foil, leaving about a 1-inch overhang on all sides, and spray the foil with nonstick cooking spray.

In a large heatproof bowl, whisk the Dutch-processed cocoa and the boiling water (½ cup + 2 tablespoons) together until smooth.

Add the finely chopped unsweetened chocolate to the cocoa mixture, let sit 30 seconds, then whisk until the chocolate is fully melted and the mixture is smooth.

Whisk in the melted unsalted butter and the vegetable oil (½ cup + 2 tablespoons). The mixture may look slightly curdled—this is normal.

Let the chocolate mixture cool for about 1 minute, then whisk in the 2 whole eggs, 2 egg yolks, and the 2 teaspoons vanilla extract until smooth and homogeneous.

Whisk in the 2½ cups granulated sugar until fully incorporated.

Add the 1¾ cups all-purpose flour and ¾ teaspoon salt. Use a rubber spatula to fold and mix just until the dry ingredients are combined—do not overmix.

Fold in the 1 cup semisweet chocolate chips until evenly distributed.

Scrape the batter into the prepared pan and spread it into an even layer.

Bake on the lowest oven rack at 350°F for 30–35 minutes, or until a toothpick inserted about halfway between the edge and the center comes out with just a few moist crumbs attached. Begin checking at 30 minutes.

Transfer the pan to a wire rack and cool for 1½ hours.

Use the foil overhang to lift the brownies from the pan, set them on the wire rack, and cool completely for about 1 more hour.

Cut into 2-inch squares and serve. Store brownies in an airtight container at room temperature for up to 4 days.