1. Preheat the oven to 350°F (175°C) and grease an 8×8 inch baking pan.
2. In a large mixing bowl, combine the melted butter, sugars, and sourdough discard. Mix until smooth.
3. Add the eggs and vanilla extract to the mixture, stirring until fully incorporated.
4. In another bowl, whisk together the cocoa powder, flour, and salt.
5. Gradually fold the dry ingredients into the wet mixture until just combined. Do not overmix.
6. If desired, fold in chocolate chips for extra richness.
7. Pour the batter into the prepared baking pan and spread evenly.
8. Bake for 20-25 minutes or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).
9. Let cool in the pan before cutting into squares and serving.