Instructions:
- Preheat oven to 350°F (175°C). Line an 8x8 inch metal baking pan with parchment paper, leaving an overhang on two opposite sides to act as handles.
- Brown the butter: Melt the butter in a light-coloured saucepan over medium heat. Continue cooking, stirring occasionally, until the milk solids turn deep amber and the butter smells nutty. Immediately pour the browned butter, including the solids, into a large heatproof mixing bowl.
- Add the semi-sweet chocolate chunks to the warm brown butter. Let it sit undisturbed for 2 minutes to soften, then whisk until the mixture is completely smooth and glossy.
- Whisk the granulated sugar and brown sugar vigorously into the chocolate-butter mixture. Add the room-temperature eggs one at a time, whisking well after each addition. Continue whisking continuously for 60–90 seconds until the batter lightens slightly and thickens. Stir in the vanilla extract.
- Sift the flour and salt over the wet ingredients. Gently fold them in using a rubber spatula until just combined; stop mixing as soon as no dry streaks remain.
- Scrape the batter into the prepared pan, smoothing the top evenly. Sprinkle with optional extra chocolate chunks. Bake for 30–35 minutes.
- Check for doneness: A skewer inserted near the center should come out with moist, fudgy crumbs attached, but not wet batter. The edges should appear set.
- Cool completely: Remove the pan from the oven and let the brownies cool fully on a wire rack for a minimum of 2 hours. This step is crucial for achieving the ultimate fudgy texture.
- Once cooled, use the parchment paper handles to lift the block from the pan, slice into 16 squares, and serve.