Prep the pan and oven: Heat the oven to 350°F (175°C). Line an 8x8-inch (20x20 cm) metal pan with parchment, leaving overhang for easy removal. Lightly grease the sides.

Brown the butter: In a medium saucepan, melt the butter over medium heat. Keep cooking, swirling often, until it foams and the milk solids turn golden and smell nutty, 3–5 minutes. Remove from heat.

Melt the chocolate: Add the dark chocolate to the hot browned butter and let it sit for 1 minute. Stir until smooth and glossy. If needed, warm briefly over very low heat to fully melt.

Whisk in cocoa: Stir in the cocoa powder until no lumps remain. This blooms the cocoa and intensifies the chocolate flavor.

Add sugars: Whisk in granulated sugar and brown sugar. The mixture will look thick and grainy—this is normal.

Beat in eggs and vanilla: Add eggs and vanilla. Whisk vigorously for 45–60 seconds until the batter looks shiny and slightly thickened. This helps create that crackly top.

Incorporate the sourdough starter: Stir in the starter until fully combined. The batter should look smooth and glossy.

Fold in dry ingredients: Sprinkle flour and salt over the batter. Fold gently with a spatula just until no dry streaks remain. Avoid overmixing to keep the brownies tender.

Add mix-ins (optional): Fold in nuts or extra chocolate chunks. Keep add-ins modest so the brownies stay ultra fudgy.

Pan and bake: Spread the batter evenly in the prepared pan. Smooth the top. Bake 24–30 minutes, until the edges are set and the center is still slightly soft. A toothpick should come out with moist, fudgy crumbs—not wet batter.

Cool completely: Set the pan on a rack and let the brownies cool fully, at least 1–2 hours. For the cleanest slices, chill for 30 minutes before cutting.

Finish: Sprinkle with flaky salt if you like. Lift out using the parchment and cut into squares with a sharp, warm knife, wiping between cuts.