Preheat your oven to 350°F (175°C) and line a square baking pan with parchment paper.
In a cast iron pan on low, melt the butter and semi-sweet chocolate chips together, stirring until smooth.8 tablespoons butter, 10 oz semi sweet chocolate chips
Add the cocoa powder and vanilla extract to the melted chocolate mixture, stirring until well combined.½ cup cocoa powder, 2 teaspoons vanilla extract
In a stand mixer with the whisk attachment, whisk together the eggs and egg yolk, cane sugar, brown sugar, and sourdough discard for 8 minutes.2 large eggs + 1 egg yolk, ¾ cup cane sugar, ½ cup brown sugar, ½ cup sourdough discard
Use the paddle attachment and add the chocolate mixture into the egg mixture and mix on low until fully incorporated.
Gradually add the all-purpose flour and a pinch of salt, mixing until the batter is smooth.1 cup all-purpose flour, pinch of salt
Pour the brownie batter into the prepared pan and smooth the top.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Let the brownies cool in the pan for about 10 minutes, then transfer them to a wire rack to cool completely.
