Preheat oven to 350 degrees (F) (175 degreed C). Line an 9×9-inch baking pan with parchment paper.
In a medium bowl, sift together the flour, baking powder, salt, and cocoa powder.
1 cup all purpose flour, 1/2 tsp baking powder, 1/2 tsp salt, 1/2 cup cocoa powder
In a medium saucepan, combine the butter, oil, and 1/4 cup of the sugar. Heat over medium heat on a stovetop, stirring frequently, until butter is completely melted, then remove from heat.
3/4 cup unsalted butter, 2 tbsp oil
In a large mixing bowl, combine the vanilla, eggs, egg yolk, and 1 cup sugar. Whisk very well, about 1 minute.
2 large eggs, 1 large egg yolk, 1.5 tsp vanilla extract
Slowly pour the warm butter mixture into the egg mixture, whisking until combined.
Add in the dry ingredients, chocolate chips, and sourdough discard. Mix until just combined, using a rubber spatula. Use care to make sure you do not over mix!
1/3 cup sourdough discard , 3/4 cup semi sweet chocolate chips
Scoop the batter into the prepared pan (it will be very thick), and smooth it with a spatula.
Bake for 27-30 minutes or until the edges are firm and a toothpick inserted comes out clean. Liberally sprinkle mini chocolate chips all over the top of the warm brownies, if using.
3/4 cup mini chocolate chips
Let cool, then slice and enjoy!