Preheat oven to 350°F and prepare a 9x9 pan with non-stick spray or parchment paper.
Roughly chop the chocolate and place into a bowl.
Slice the butter into smaller pieces and add to the chocolate. Microwave for 1 minute. Stir well and then microwave for another 30 seconds. Stir again and stir until all pieces are melted and the butter is incorporated.
Add sugar to the chocolate and butter. Stir well.
Add in vanilla, eggs, and pinch of salt. Stir until well incorporated.
Add sourdough starter and stir by hand until smooth.
Pour brownie batter into prepared pan and bake for 25-28 minutes or until an inserted toothpick is removed with a few crumbs but no wet batter. Do not overbake. (Every oven is different so start checking at 22 minutes just in case!)