Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a medium bowl, combine the unsweetened cocoa powder and boiling water. Stir well until the mixture is smooth and glossy. Set it aside to cool slightly.
In a separate bowl, melt the chopped unsweetened chocolate in the microwave in 30-second intervals, stirring between each interval until fully melted. Allow it to cool slightly.
In a large mixing bowl, whisk together the melted butter, vegetable oil, eggs, egg yolks, and vanilla extract until well combined. Add the cooled cocoa mixture and melted chocolate, mixing until smooth.
Sprinkle the flour, granulated sugar, and salt over the wet ingredients. Gently fold the dry ingredients into the wet mixture until just combined. Be careful not to overmix!
Fold in the semi-sweet chocolate chunks, ensuring they're evenly distributed throughout the batter.
Pour the brownie batter into the prepared pan, spreading it evenly. Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for about 10 minutes, then use the parchment overhang to lift them out. Let them cool completely on a wire rack before cutting into squares.