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

Melt the butter: In a small saucepan or microwave-safe bowl, melt 10 tablespoons of butter until just liquid, not sizzling. Stir in 2 tablespoons of neutral oil. Let it cool for 3–4 minutes.

Whisk the dry ingredients: In a bowl, whisk 3/4 cup all-purpose flour, 1/2 cup unsweetened cocoa powder, 1/2 teaspoon fine sea salt, 1/4 teaspoon baking powder, and 1 teaspoon espresso powder (if using). Break up any cocoa lumps.

Mix the sugars and eggs: In a separate bowl, whisk 3/4 cup granulated sugar and 1/2 cup light brown sugar with 2 large eggs plus 1 yolk for 45–60 seconds until slightly thickened and glossy. Whisk in 2 teaspoons vanilla.

Add the butter: Slowly stream the warm butter-oil mixture into the egg-sugar mixture while whisking. This helps create that shiny, crackly top.

Fold in the dry mix: Switch to a spatula. Add the dry ingredients and fold gently just until no dry streaks remain. The batter will be thick and glossy. Do not overmix.

Pan and smooth: Scrape the batter into the lined pan and smooth the top with an offset spatula or the back of a spoon.

Bake: Bake for 22–28 minutes. You want set edges and a center that looks slightly underdone. A toothpick should come out with a few moist crumbs, not wet batter.

Cool and set: Let the pan cool on a rack for at least 1 hour. For the cleanest slices and a fudgy texture, chill for 30 minutes after cooling, then lift out and cut.

Serve: Cut into 9 large squares or 16 smaller ones. Enjoy at room temperature or slightly warm with a scoop of ice cream.