Bloom the Cocoa Powder: Combine the coffee and cocoa powder in a heat-proof bowl. Set aside. This intensifies the chocolate flavor in the cocoa powder.
Make the Batter: Place a small saucepan filled with 2 inches of water over medium heat. Add the butter, vegetable oil, semisweet and bittersweet chocolate into the bowl with the cocoa powder mixture.
Place the bowl over the simmering water (you’ll want to make sure the water does not touch the bottom of your bowl). Gently melt the butter and chocolate, using a spatula stir the mixture occasionally until the mixture is smooth. Remove the bowl from the heat and allow the chocolate mixture to cool slightly.
Add the granulated sugar and brown sugar into the chocolate mixture, whisk vigorously until the sugars have dissolved and the oil is fully emulsified into the mixture. Add the egg, egg yolks and vanilla into the chocolate mixture, whisk vigorously for at least 1 minute or until your mixture looks shiny and smooth. Stir in the sourdough discard until it has fully dissolved and no traces of starter remain.
Stir the flour, cornstarch and salt into the batter and mix vigorously for at least 1 minute, this will help develop a little bit of gluten into your mixture and contribute to the chewy texture of your brownies.
Using a wooden spoon or a spatula, fold in chocolate chips and walnuts into your batter. Set aside while you preheat the oven.
Bake your Brownies: Preheat your oven to 350 F. Line a 8-inch square baking pan with aluminum foil leaving an overhang and pressing the foil into the corners. Brush the foil with vegetable oil and set aside.
Using a flexible spatula, scrape the batter into your baking pan. Bake the brownies for 25 to 30 minutes or until the surface looks dry and glossy. You’ll know the brownies are done when the edges begin to pull away from the pan, and the center is a little puffy but still soft to the touch.