Preparation Steps

• Heat the oven to 325°F (165°C). Line a 9×13-inch metal baking pan with a parchment sling, leaving overhang on the long sides, and lightly grease the paper.
• In a medium bowl, whisk together the flour, cornstarch, baking soda, fine sea salt, and espresso powder (if using); set aside.
• Melt 6 tablespoons (85g) of the butter in a small saucepan over medium heat just until fully melted. Off the heat, whisk in the brown sugar until glossy and combined. Let cool until just warm, 5 minutes.
• In a stand mixer (or with a hand mixer), beat the remaining butter with the granulated sugar on medium speed until creamy, 45–60 seconds. Scrape the bowl, pour in the cooled brown sugar–butter mixture, and mix on low to combine, about 30 seconds.
• Beat in the eggs and yolk one at a time on low speed, mixing just until each disappears. Stir in the vanilla.
• Add the dry ingredients in two additions, mixing on low only until a few dry streaks remain. Finish folding by hand with a spatula to avoid overmixing; the dough will be thick.
• Fold in about 1 1/2 cups (255g) of the chocolate chips, reserving the rest for the top.
• Press roughly two-thirds of the dough into the prepared pan in an even layer. Pinch off pieces of the remaining dough and scatter over the surface. Sprinkle on the reserved chocolate and lightly press to adhere.
• Chill the filled pan for 10–15 minutes to help the bars bake up extra thick.
• Bake on the center rack for 25–30 minutes, rotating the pan halfway, until the edges are deeply golden and the center looks set but still slightly soft. If the top is browning too quickly, tent loosely with foil for the last few minutes.
• Cool the pan on a rack for 10 minutes, then sprinkle with flaky sea salt (if using). Let cool completely in the pan before lifting out with the parchment sling and cutting into 24 bars.
• Practical tip: For the chewiest texture, weigh your flour (about 440g) or spoon and level—too much flour makes bars cakey.