Prep the pan and oven: Heat oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Melt the butter and chocolate: In a heatproof bowl, melt butter and chopped chocolate together over a gentle simmer (double boiler) or in the microwave in 20–30 second bursts. Stir until smooth. Whisk in the oil. Let cool 3–4 minutes so it’s warm, not hot.

Whisk sugars and eggs: In a separate bowl, whisk granulated sugar, brown sugar, eggs, and the extra yolk for 1–2 minutes until slightly thick and glossy. This step helps create a shiny top.

Combine wet ingredients: Whisk vanilla into the egg mixture. Stream in the warm chocolate-butter mixture while whisking until fully combined.

Mix dry ingredients: In a small bowl, whisk flour, cocoa powder, fine sea salt, and espresso powder (if using). Break up any cocoa lumps.

Fold together: Add dry ingredients to the wet. Using a spatula, fold gently just until no dry spots remain. Do not overmix. The batter will be thick and glossy.

Pan and smooth: Scrape batter into the prepared pan and spread evenly. Tap the pan once on the counter to pop large air bubbles.

Bake: Bake 22–28 minutes, depending on your oven. Pull them when the edges are set, the top is shiny and crackly, and a toothpick comes out with moist crumbs, not wet batter. Err on the side of slightly underbaked for maximal chew.

Cool completely: Place the pan on a rack and cool at least 1 hour. For the cleanest slices and best texture, chill 30 minutes after cooling. Lift out using the parchment and cut with a warm, sharp knife.

Finish (optional): Sprinkle with flaky sea salt just after baking for a sweet-salty snap.