Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking pan.
In a large mixing bowl, combine the melted butter and sugar. Beat them together until smooth. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract and sourdough starter.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients, folding gently until just incorporated. Be careful not to overmix.
If using, fold in the chocolate chips.
Pour the batter into the prepared pan, smoothing it out evenly. Bake for 25-30 minutes, until a toothpick inserted in the center comes out clean with just a few moist crumbs.
Allow the brownies to cool completely in the pan before cutting into squares.
Notes
These brownies are rich and fudgy. Store them in an airtight container at room temperature for up to 4 days or freeze them for up to 3 months.