Preheat oven to 325°F. Butter an 8-9 inch cast iron skillet, 8-inch cake pan, or 8x8-inch square pan.
In a medium saucepan, melt butter over medium-low heat. When melted, add sugar and stir until combined (don't worry about the sugar fully dissolving). Remove from heat.
In a medium bowl combine flour, cocoa powder, and salt. Whisk together.
In a small bowl combine eggs, oil, vanilla, and sourdough discard. Whisk until combined.
Add wet ingredient mixtures to dry ingredient mixture. Stir with a rubber spatula or wooden spoon, adding chocolate chips about halfway through mixing. Mix until just combined, try to avoid overworking the batter.
Transfer to buttered pan and spread into an even layer with a rubber spatula.
Bake for about 25-35 minutes or until brownie appear set. A toothpick inserted in the center should come out with a few moist crumbs. Allow to cool completely (at least an hour) before slicing and serving.