Preheat your oven to 350°F (175°C). Line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang on two sides to easily lift the brownies out later. Lightly grease any exposed pan areas if desired.

In a medium saucepan or a microwave-safe bowl, melt the unsalted butter. Once melted, remove from heat. Add the granulated sugar and light brown sugar to the melted butter and whisk vigorously for about 1 minute until well combined and glossy. This step helps dissolve the sugar, contributing to the fudgy texture.

Transfer the butter-sugar mixture to a large mixing bowl. Whisk in the eggs one at a time until fully incorporated, then stir in the vanilla extract. The mixture should be thick and shiny. Add the room temperature sourdough discard and whisk until just combined. Don't overmix.

In a separate small bowl, whisk together the all-purpose flour, unsweetened cocoa powder, and salt. Sift the dry ingredients over the wet ingredients in the large mixing bowl. Using a rubber spatula, gently fold the dry ingredients into the wet until just combined and no streaks of flour remain. Be careful not to overmix, as this can lead to tougher brownies. If using, fold in the chocolate chips or chunks now.

Pour the brownie batter into the prepared baking pan and spread evenly. If desired, sprinkle a few extra chocolate chips on top. Bake for 25-28 minutes, or until a toothpick inserted into the center comes out with moist crumbs, but not wet batter. The edges should look set, but the center might still look slightly underbaked – this is key for fudginess.

Remove the pan from the oven and let the brownies cool completely in the pan on a wire rack for at least 2 hours, or preferably longer (even overnight). Cooling completely is crucial for the fudgy texture to set. Once cool, use the parchment paper overhang to lift the brownies out of the pan, slice into squares, and serve. Enjoy!