Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving some overhang on the sides for easy removal later.
In a medium bowl, whisk together the unsweetened cocoa powder and boiling water until smooth. This step is crucial as it helps to intensify the chocolate flavor.
In a separate bowl, combine the chopped unsweetened chocolate and melted butter. Stir until the chocolate is completely melted and the mixture is smooth.
Add the cocoa mixture to the chocolate and butter blend. Stir in the vegetable oil, eggs, egg yolks, and vanilla extract until fully combined.
Gradually mix in the granulated sugar, ensuring it is well combined with the wet ingredients.
In another bowl, whisk together the all-purpose flour and salt. Gradually fold this mixture into the wet ingredients using a spatula, being careful not to overmix.
Gently fold in the semi-sweet chocolate chunks until evenly distributed throughout the batter.
Pour the brownie batter into the prepared baking pan, spreading it evenly. Bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for at least 10 minutes before lifting them out using the parchment paper. Let them cool completely on a wire rack before slicing into squares.
