Preparation Steps

• Preheat the oven to 325°F (165°C). Lightly grease a 9×13-inch metal pan and line it with a parchment sling that overhangs the long sides for easy lifting.
• In a medium bowl, whisk together the flour, cornstarch, baking soda, fine sea salt, and espresso powder until evenly combined.
• In a large mixing bowl or the bowl of a stand mixer fitted with the paddle, beat the softened butter with the brown sugar on medium speed for about 1 minute. Sprinkle in the granulated sugar and continue mixing for another 1 minute, just until creamy. Scrape the bowl.
• Add the eggs one at a time, mixing on low after each until incorporated. Blend in the vanilla. Do not overbeat—you want a smooth, slightly thick batter, not fluffy.
• Add half of the dry ingredients and mix on low just until a few dry streaks remain. Add the rest and mix briefly; finish folding by hand with a spatula to avoid overworking the dough.
• Fold in most of the chocolate chips, reserving a small handful for the top. Scrape the dough into the prepared pan and spread it into an even layer. Scatter the reserved chips over the surface and gently press them in.
• Chill the filled pan in the refrigerator for 10 minutes. This short rest helps the bars bake up thicker with a fudgy center.
• Bake on the center rack for 27 to 33 minutes, until the edges are set and deeply golden and the center looks slightly puffed with a few moist crumbs clinging to a tester. (If using a glass pan, add 3 to 5 minutes.)
• Cool the pan on a rack for 15 minutes, then sprinkle lightly with flaky sea salt. Use the parchment sling to lift the slab out and cool completely before slicing into bars.