Preheat your oven to 350°F (175°C). Line a 9x9 inch baking pan with parchment paper, leaving an overhang on two sides to easily lift the brownies out later. Lightly grease the parchment paper.

In a medium saucepan or a microwave-safe bowl, melt the unsalted butter. Remove from heat and stir in the granulated sugar and light brown sugar until well combined. The mixture should be warm, but not scorching hot.

Transfer the butter-sugar mixture to a large mixing bowl. Whisk in the 2 large eggs and 1 large egg yolk, one at a time, until fully incorporated and the mixture is glossy. Stir in the vanilla extract.

In a separate medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, and salt until there are no lumps.

Gradually add the dry ingredients to the wet ingredients, mixing with a rubber spatula until just combined. Be careful not to overmix; overmixing can lead to dry, cakey brownies. Stop mixing as soon as no streaks of flour remain.

Gently fold in the semisweet chocolate chips or chopped chocolate chunks until evenly distributed throughout the batter.

Pour the brownie batter into the prepared 9x9 inch baking pan and spread evenly.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs, but not wet batter. The edges should be set, and the center may still look slightly underbaked – this is key for fudginess!

Remove the brownies from the oven and let them cool completely in the pan on a wire rack for at least 2-3 hours, or even longer (ideally, overnight). This allows them to set properly and enhances their fudgy texture. Once cool, use the parchment paper overhang to lift them out of the pan and cut into 16 squares.