Prep the pan: Line an 8-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Preheat: Set the oven to 350°F (175°C). Let eggs sit out to come to room temperature if you haven’t already.

Melt butter and chocolate: In a heatproof bowl, melt the butter with 6 oz (170 g) chopped semi-sweet chocolate. Use a microwave in 20–30 second bursts, stirring between, or a double boiler. Stir until smooth and glossy.

Whisk in sugars: While the mixture is still warm, whisk in the granulated and brown sugar until well combined and slightly thickened. This helps create the crackly top.

Add eggs and vanilla: Whisk in eggs one at a time, mixing for about 20–30 seconds after each, until the batter looks shiny and cohesive. Whisk in vanilla. The batter should look thick and glossy.

Dry ingredients: Sift or whisk together flour, cocoa powder, salt, and espresso powder (if using). Add to the wet mixture and fold gently with a spatula just until no dry streaks remain. Do not overmix.

Fold in chunks: Stir in 4–6 oz (113–170 g) of chopped chocolate. Reserve a small handful to sprinkle on top.

Pan and top: Spread batter evenly in the pan. Sprinkle the reserved chocolate and a pinch of flaky salt (if using) over the surface.

Bake: Bake 24–30 minutes, depending on your oven and how fudgy you like them. The center should be set but still soft, with a few moist crumbs on a toothpick. Don’t wait for a clean pick.

Cool and set: Cool in the pan on a rack for at least 1 hour. For the cleanest slices and maximum fudginess, chill for 30–60 minutes after cooling, then lift out and cut with a warm, sharp knife.