Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.
In a large bowl, mix the melted butter with both sugars until well combined.
Add the eggs, one at a time, mixing well after each addition.
Stir in the vanilla extract and sourdough starter until smooth.
In a separate bowl, whisk together the cocoa powder, flour, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the chocolate chips if using.
Pour the batter into the prepared baking pan and spread evenly.
Bake for 25-30 minutes, or until a toothpick inserted comes out with a few moist crumbs.
Allow to cool in the pan before cutting into squares.