Preparation Steps

• Heat the oven to 350°F. Line a rimmed 18×13-inch sheet pan with parchment, leaving overhang on the long sides for lifting. Lightly grease any exposed metal. For thicker brownies, fold a double layer of foil to form a 1-inch dam along one long and one short edge to slightly reduce the baking area.
• Whisk the dry ingredients: In a medium bowl, whisk together the flour, cocoa, espresso powder, and fine sea salt until evenly combined; set aside.
• Melt the chocolate and butter: Place the butter and bittersweet chocolate in a heatproof bowl set over a pan of barely simmering water. Stir until smooth and fully melted, then remove from heat and stir in the vanilla. Keep warm to the touch.
• Beat eggs with sugars: In a large bowl, whisk the eggs, granulated sugar, and brown sugar for 1 to 2 minutes until slightly thickened and the mixture looks glossy (no need to whip to a ribbon). Tip: For an extra shiny, crinkly top, set the bowl over warm—not hot—water while whisking to help dissolve the sugars.
• Emulsify: While whisking constantly, slowly stream the warm chocolate mixture into the egg-sugar mixture until smooth and uniform.
• Fold in the dry ingredients in two additions with a spatula just until no dry streaks remain. Fold in about two-thirds of the chopped dark chocolate; do not overmix.
• Rest the batter for 10 minutes to thicken and hydrate the cocoa. Scrape the batter into the prepared pan, smooth the top, and tap the pan firmly on the counter 2 to 3 times to release air bubbles. Sprinkle the remaining chopped chocolate over the surface.
• Bake for 18 to 22 minutes, rotating the pan once at 12 minutes, until the edges are set, the center is slightly soft, and a toothpick comes out with a few moist crumbs (not wet batter).
• Cool on a rack for at least 20 minutes. Sprinkle a pinch of flaky sea salt over the warm brownies if using, lift out using the parchment overhang, and cut into 24 squares.