Preheat your oven to 350°F. Lightly grease a 9×13-inch baking pan with nonstick spray, or line it with parchment paper for easy removal.

In a small bowl, whisk together the flour, granulated sugar, and salt. Set aside.

In a large mixing bowl (or the bowl of a stand mixer), whisk the cocoa powder and boiling water until smooth and no lumps remain.

While the cocoa mixture is still hot, stir in the chopped unsweetened chocolate. Continue stirring until the chocolate is fully melted and the mixture is smooth.

Add the vegetable oil, melted butter, eggs, egg yolks, and vanilla extract to the bowl. Whisk until fully combined and the batter looks smooth and glossy.

Gently fold the dry ingredients into the wet batter using a spatula. Stir just until no dry streaks remain—avoid overmixing.

Pour the batter into your prepared pan. Use the back of a spoon or spatula to smooth out the surface evenly.

Bake for 30–35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (but not wet batter).

Let the brownies cool in the pan before slicing. Enjoy warm, chilled, or straight from the pan with a fork.