Step 1: Preheat your oven to 350°F (175°C). Grease and flour an 8x8 or 9x9 inch baking pan, or line with parchment paper, leaving an overhang on two sides for easy removal.
Step 2: In a large mixing bowl, combine the melted butter, granulated white sugar, and packed dark brown sugar. Whisk until well combined and smooth.
Step 3: Add the large eggs one at a time, whisking well after each addition until fully incorporated. Stir in the pure vanilla extract.
Step 4: Sift the all-purpose flour, Dutch-processed (or unsweetened) cocoa powder, espresso powder (if using), and kosher salt directly into the wet ingredients in the bowl. Gently fold with a spatula until just combined. Be careful not to overmix; a few streaks of flour are fine.
Step 5: Fold in 1 cup of the chopped chocolate into the batter.
Step 6: Pour the batter into your prepared baking pan and spread evenly. Sprinkle the remaining ¼ cup of chopped chocolate over the top of the batter.
Step 7: Bake for 25-30 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs, not wet batter. The center should still look a little fudgy.
Step 8: Remove from the oven and let cool completely in the pan on a wire rack before cutting into squares. This is crucial for fudgy brownies!