Prep the pan and oven. Heat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, letting it hang over two sides for easy lifting. Lightly grease the exposed sides.

Melt the butter and chocolate. Place butter and chopped chocolate in a heatproof bowl. Melt gently over a saucepan of barely simmering water, stirring until smooth, or microwave in short bursts. Stir in the oil. Set aside to cool slightly.

Whisk sugars and eggs. In a separate bowl, whisk granulated sugar, brown sugar, eggs, and the extra yolk for about 1 minute until slightly thick and glossy. This helps create the crackly top. Whisk in vanilla and espresso powder.

Combine wet ingredients. Slowly stream the warm chocolate mixture into the egg mixture while whisking. Mix until smooth and uniform.

Add dry ingredients. Sift or whisk together flour, cocoa powder, and salt. Fold into the batter with a spatula just until no dry streaks remain. Do not overmix. If using, fold in chocolate chips or chunks.

Pan and smooth. Scrape the batter into the prepared pan and smooth the top. If you like, sprinkle lightly with flaky sea salt.

Bake. Bake for 22–28 minutes, depending on your oven. The edges should look set and slightly pulled from the sides, while the center should look glossy and a bit underdone. A toothpick should come out with some moist crumbs, not wet batter.

Cool and slice. Let the brownies cool in the pan on a rack for at least 30–45 minutes for clean slices. For the fudgiest texture, chill for 30 minutes before cutting, then slice with a warm, clean knife.

Serve. Enjoy at room temperature or slightly warm. If you chilled them, let them sit for 10 minutes before serving to soften a bit.