Preheat oven to 350℉. Spray an 9x9-inch baking pan with cooking spray, then line with parchment paper (this makes it easier to remove the baked brownies). Set aside.
In a small saucepan over medium-low heat, melt the butter. Add the chocolate chips, oil, and vanilla extract, and stir until melted and combined. Remove from heat and whisk in the cocoa powder until smooth. Set aside to cool slightly.
In the bowl of a stand mixer fitted with a whisk attachment*, beat the eggs and sugars until light and fluffy, and the sugar has dissolved (about 8-10 minutes). Don't overmix during this part: Using a spatula, fold the sourdough discard and chocolate mixture into the whipped egg mixture. Add the flour and salt and mix until just combined. The mixture wil be thick.
Spread the batter into your prepared baking pan and bake for 22-25 minutes. When fully baked, the brownies should be set on the edges, but may still be a bit soft in the center.
After baking, let the brownies cool in the pan for at least 30 minutes. They will continue to bake internally during this time, and the shiny crust will develop as the brownies cool. Remove the brownies from the pan and let cool fully. Slice into 25 pieces before serving.
Notes
*I highly recommend making this recipe with a stand mixer, but you can also use a hand-held mixer. Be prepared that you will be mixing the eggs and sugar for a long time!To make this recipe with active sourdough starter, replace the sourdough discard with active starter (150g). You do not need to change any other ingredients in the recipe. Serving size = 1 brownie.