Start by preheating your oven to 375°F (190°C). Grease an 8x8-inch metal baking pan with nonstick spray or butter, then line it with parchment paper, leaving an overhang on all sides.

In a small saucepan, melt the butter over medium heat until it begins to foam and bubble. Whisk frequently as it cooks, and keep a close eye on it as it starts to turn golden brown and smells nutty—usually about 3-5 minutes. Once browned, pour the butter into a large mixing bowl, scraping in all the browned bits for added flavor.

While the butter is still warm, whisk in the cocoa powder and vanilla extract until smooth. The heat from the butter will help bloom the cocoa, intensifying the chocolate flavor in the final brownies.

Add the granulated sugar and salt to the mixture, stirring well until fully incorporated. This helps dissolve the sugar, creating a smoother batter.

Add the eggs one at a time, whisking vigorously after each addition. The batter may look separated at first, but after beating in the second egg, it will transform into a smooth, glossy mixture. This step is essential for achieving that iconic brownie texture.

Gently fold in the sourdough starter and discard until well mixed. Then, fold in about two-thirds of the chopped bittersweet chocolate, reserving the rest for sprinkling on top.

Pour the batter into the prepared pan, smoothing it into an even layer. Sprinkle the remaining chocolate chunks on top, then bake for 29-32 minutes.

Immediately sprinkle the brownies with flaky sea salt, then let them cool in the pan for at least 35 minutes. For the best texture, allow them to cool for an hour, then chill in the refrigerator for another 15 minutes before slicing.