Preheat your oven to 350°F (175°C). Grease or line a 9x13-inch baking pan with parchment paper.

In a medium saucepan, melt the unsalted butter over low heat. Remove from heat and stir in granulated sugar and brown sugar until combined.

Allow the mixture to cool slightly, then add eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Mix in your sourdough starter until everything is well blended.

In a separate bowl, whisk together all-purpose flour, cocoa powder, baking powder, and salt.

Gradually fold the dry ingredients into the wet ingredients until just combined. If using, fold in chocolate chips.

Pour the batter into the prepared baking pan and spread it evenly. 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 before transferring to a wire rack to cool completely. Cut into squares and enjoy.