Preheat your oven to 350°F (175°C). Grease the baking pan or line it with parchment paper for easy removal.
In a mixing bowl, melt the unsalted butter. Let it cool slightly before proceeding.
Once cooled, whisk in the granulated sugar and brown sugar until fully combined.
Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
Mix in the sourdough starter until the mixture is smooth.
In another bowl, combine the flour, cocoa powder, baking powder, and salt. Gradually add this dry mixture to the wet mixture, folding gently with a rubber spatula until just combined.
If using, fold in the chocolate chips to distribute them throughout the batter.
Pour the brownie batter into the prepared baking pan, spreading it evenly.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Remove from the oven and let the brownies cool in the pan for about 10 minutes before transferring them to a wire rack to cool completely.
Slice into 9 squares and enjoy!