Preparation Steps

• Heat the oven to 350°F. Line a 13×18-inch rimmed sheet pan with parchment, leaving overhang on the long sides for easy lifting. Lightly coat the parchment and sides with nonstick spray or a thin layer of butter.
• Brown the butter: Place the butter in a medium saucepan over medium heat. Cook, stirring, until the milk solids turn golden and smell nutty, 4–6 minutes. Remove from the heat and immediately stir in the oil.
• Melt the chocolate: Add 12 oz of the chocolate to the warm browned butter and let sit for 2 minutes. Whisk until smooth, then stir in the remaining 6 oz until glossy. Set aside to cool to lukewarm—warm to the touch but not hot.
• Whip the eggs and sugar: In a stand mixer fitted with the whisk (or with a hand mixer), beat the eggs, granulated sugar, vanilla, espresso powder, and fine salt on medium-high until very thick, pale, and ribbony, about 5–7 minutes. This aeration gives you shiny tops and a fudgy center.
• Mix in the chocolate: With the mixer on low, slowly stream in the lukewarm chocolate-butter mixture until combined. Scrape the bowl well.
• Combine the dry ingredients: In a separate bowl, whisk the flour, cocoa powder, and cornstarch until evenly blended. Add to the batter and fold gently with a spatula just until no dry streaks remain. If using, fold in the extra chocolate chips or chunks.
• Bake: Spread the thick batter evenly into the prepared sheet pan and smooth the surface. Sprinkle lightly with flaky sea salt if you like. Bake until the top is shiny and a toothpick inserted toward the center comes out with a few moist crumbs, 24–28 minutes. Do not overbake.
• Cool and slice: Cool completely in the pan on a rack. For the cleanest cuts and extra-fudgy texture, chill for 30 minutes before slicing. Lift out using the parchment and cut into 24 squares with a hot, wiped knife or a plastic knife.