Preparation Steps

• Line a 9×13-inch metal baking pan with parchment, leaving overhang on two sides for a sling. Lightly butter the parchment and sides. Preheat the oven to 345°F with a rack in the center.
• Place 10 ounces of the chopped chocolate and the butter in a large microwave-safe bowl. Microwave in 30-second bursts, stirring between each, until just melted and smooth, 1½–2 minutes total. Stir in the hot brewed espresso until glossy. Set aside; keep warm but not hot.
• In a separate large bowl, whisk together the granulated sugar, brown sugar, eggs, egg yolk, vanilla, and molasses for about 2 minutes until thickened and slightly lighter. Tip: This brief whisk helps create that shiny, crackly top.
• While whisking the sugar-egg mixture, slowly stream in the warm chocolate-butter mixture until fully combined and smooth.
• In a medium bowl, whisk the flour, cocoa powder, and salt to break up any lumps. Fold this dry mixture into the chocolate base with a spatula just until a few streaks remain. Fold in the remaining 2 ounces of chopped chocolate so some bits stay chunky.
• Scrape the batter into the prepared pan, smooth the surface, and tap the pan firmly on the counter 2–3 times to release any large air bubbles. Let the filled pan rest on the counter for 10 minutes to hydrate the flour and deepen the crackly crust.
• Bake for 24–28 minutes, rotating the pan once halfway through. The edges will be set and the center should yield moist, fudgy crumbs on a toothpick (not wet batter).
• Set the pan on a wire rack and immediately sprinkle with a pinch of flaky sea salt, if using. Cool completely in the pan. For the cleanest slices, chill for 30 minutes, then lift out using the sling and cut into 24 squares with a hot knife wiped clean between cuts.