Fill a pot with 2 inches of water and bring it to a boil on the stovetop.
In a heat-safe glass mixing bowl, add butter and chocolate chips.
Once the water is boiling, reduce the heat to low-medium. Place the bowl on top of the pot, ensuring no steam escapes, and the bowl doesn't touch the water.
Stir until the chocolate and butter are melted and well combined (double boiler method).
Carefully remove the bowl from the pot and place it on a hot pad (it will be hot).
Preheat the oven to 350 degrees.
In the bowl, add cane sugar, eggs, vanilla, sourdough starter, cocoa powder, flour, and salt. Mix until combined, using a whisk.
Stir in an extra 1/4 cup of chocolate chips and set the batter aside (and chopped walnuts or pecans if you prefer)
Line a 9-inch round cake pan with parchment paper or lightly grease it with butter.
Pour the batter into the pan and bake for 17-20 minutes (until a toothpick comes out clean when poked in the center of the brownies).
Slice the brownies into squares and enjoy!