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

Melt butter and chocolate. In a microwave-safe bowl or over a double boiler, melt the butter and semi-sweet chocolate. Stir until smooth, then let it cool slightly so it’s warm, not hot.

Whisk the sugars and eggs well. In a large bowl, whisk granulated sugar, brown sugar, eggs, and egg white for 1–2 minutes until slightly thick and glossy. This step helps create the crackly top.

Add vanilla, oil, and melted chocolate mixture. Whisk in vanilla, then slowly whisk in the oil and the melted butter-chocolate mixture until smooth and uniform.

Sift and mix the dry ingredients. In a separate bowl, whisk flour, cocoa powder, baking powder, and salt. Sifting the cocoa is helpful if it’s clumpy.

Combine wet and dry. Add the dry ingredients to the wet in two additions, alternating with the milk. Stir gently with a spatula just until no dry streaks remain. Do not overmix.

Fold in add-ins (optional). If using mini chips or nuts, fold them in now. Keep the batter airy.

Pan and smooth. Spread the batter evenly in the prepared pan. Tap the pan once on the counter to settle the batter and release large air bubbles.

Bake. Bake for 22–28 minutes, until a toothpick inserted near the center comes out with a few moist crumbs. The top should look shiny and lightly crackled.

Cool and slice. Cool in the pan for at least 30 minutes. Lift out with the parchment and cool fully before slicing into 9 or 16 squares with a sharp knife. Wipe the knife between cuts for clean edges.