Preheat the oven to 350°F and line a 13 x 9 inch baking pan with parchment paper or spray it with non stick spray. 

Place the butter and sugar in a saucepan and heat over medium heat until the better melts. Continue to cook over medium heat for 1 to 2 minutes (this will help some of the sugar dissolve and give the brownies a shiny top). 

Remove from heat and toss in the chopped chocolate and cocoa powder. Stir until all the chocolate melts. 

Transfer the chocolate mixture to a large mixing bowl and stir for a few minutes until it cools down slightly. 

Stir in the sourdough starter, eggs, and vanilla extract. Place a sifter over the bowl and sift the flour, salt, espresso powder, and baking powder over the batter. Stir until well combined. Stir in the chocolate chips. 

Spread the batter into the 13 x 9 inch pan and bake for 22 to 28 minutes or until the edges look set. If you have an instant read thermometer the center of the brownies will be 185°F to 198°F when done.