Preheat oven to 350°F (176°C). Line a 9x9-inch baking pan with parchment paper, allowing some overhang on the sides.

In a saucepan over low heat, melt the butter. Add in chocolate chips, vanilla, and cocoa powder, stirring until smooth. Remove from heat and let cool slightly.

In a large mixing bowl, beat the eggs, egg yolk, granulated sugar, and brown sugar together using a mixer on high speed for 7 to 10 minutes until light and creamy.

Reduce mixer speed to low and blend in the sourdough discard and the slightly cooled chocolate mixture until well combined.

Gently fold in the flour and salt using a spatula, mixing just until the dry ingredients are incorporated. The batter will be thick.

Spread the batter evenly in the prepared pan.

Bake for 35–40 minutes, or until a toothpick inserted in the center comes out with moist crumbs but not wet batter.

Let the brownies cool completely in the pan. Use the parchment to lift them out, then cut into squares and serve.