Prep. Spray a 9 x 13 baking dish with nonstick spray. Then, preheat the oven to 350°F (175°C) with the rack in the center.
Combine all ingredients. Mix the vegetable oil, cocoa powder, all-purpose flour, granulated sugar, salt, eggs, sourdough starter, and vanilla extract in a large mixing bowl until smooth and shiny. You can use a spatula or a hand mixer. If you're using a mixer, add the chocolate chips last.
Bake. Pour the batter into the prepared baking dish, then bake for 20-25 minutes. Don't forget to check for doneness at 20 minutes. The brownies are ready when you insert a toothpick in the center, and it comes out with large, moist crumbs but no wet batter. Even if they look slightly underdone, they will firm up as they cool.
Serve. After taking the brownies out of the oven, let them cool in the pan for at least 20 minutes. Then, cut them into squares and serve them straight from the pan.