Preheat oven to 375 degrees. Butter or oil a 9x9 inch pan.
Put eggs and egg white, and sugar and brown sugar in a large bowl and whip until foamy. Then add water and vanilla and whip until the mixture is entirely combined with the sugar completely dissolved.
In a small sauce pot, add butter, oil, salt, and chocolate and stir over low heat until it is melted then remove from heat. Add cocoa and mix until combined. Then add sourdough starter and stir until combined.
Add your chocolate mixture to your egg mixture, gently folding it in until combined.
Bake the brownies for 25 minutes, until the top is firm. The inside will still be soft, you can bake it for longer if you don't want a gooey brownie.