Preheat your oven to 350°F (175°C). Take your 9x13 inch baking pan and give it a good spray with nonstick cooking spray. For extra security, line the bottom with parchment paper, allowing it to hang over the edges. This little trick will make it easy to lift the brownies out after baking. Give the parchment a light spray as well, and set the pan aside.

In a large mixing bowl, whisk together the melted, slightly cooled butter, granulated sugar, and dark brown sugar. Ensure that the sugars are fully dissolved into the butter, giving you a smooth mixture. This is where the magic happens!

Next, crack in four large eggs one by one, adding them to the mixture along with the tablespoon of pure vanilla extract. Whisk them thoroughly until the mixture is smooth and well combined. This will enhance the flavor profile of your brownies.

Sift in the all-purpose flour and cocoa powder directly into the bowl. Mixing them in this way helps to avoid clumps in your batter. Add in the kosher salt and espresso powder if you’re using it. Gently stir the mixture until just combined; be careful not to overmix—this is the key to achieving those perfect, fudgy brownies.

Now, grab your chocolate chunks. Fold in 1 cup of them to the brownie batter, deeply incorporating these sweet morsels. You'll be thrilled by the chocolatey goodness in each bite! Once mixed, gently pour the batter into your prepared pan, spreading it out evenly. Top this with the remaining ¼ cup of chopped chocolate to give a delightful finish.

Place your pan on the middle rack of your preheated oven and bake for 25-30 minutes. The brownies are done when they are just set to touch. Remember, they will firm up a bit as they cool, so don’t overbake. Once out of the oven, allow them to cool completely in the pan before lifting them out using the parchment overhang. After cooling, cut them into squares. If you like, sprinkle a touch of flaky sea salt on top for that extra flavor burst.