Evenly grease an 8″ sq. pan, then line with parchment and calmly grease the parchment. Preheat the oven to 350°F with a rack inside the center.
In a giant saucepan, heat the butter, granulated sugar, and brown sugar over medium heat, stirring typically to encourage even melting, until the mixture could also be very scorching (the butter must be effervescent throughout the perimeters of the pan) and the sugar is starting to dissolve, 3 to 5 minutes.
Take away the pan from the heat and immediately whisk inside the cocoa, vanilla, and salt, whisking until the mixture is simple. Let cool for 5 minutes (the mixture and pot should nonetheless be pretty warmth), then add the eggs individually, whisking vigorously after each addition. The mixture might look greasy and broken after the first egg, nevertheless it might flip simple and glossy after the second.
Whisk inside the sourdough starter and baking powder until no streaks of starter keep, then vigorously stir in 1/2 cup (85g) chocolate chips, encouraging them to melt into the good and comfy batter.
Pour the batter into the prepared pan and sprinkle with the remaining 3 tablespoons (32g) chocolate chips.
Bake the brownies for 22 to 26 minutes, until the very best is shiny nevertheless not wet-looking, the perimeters are starting to attract again from the perimeters of the pan, and an inserted toothpick comes out with no raw batter.
Let the brownies cool completely inside the pan, about 1 hour. For the only slicing, cowl the cooled brownies and chill inside the fridge for no less than 1 hour.
Brownies keep best (and magnificence best!) wrapped tightly and saved inside the fridge, the place they’ll closing for as a lot as 1 week, or the freezer, the place they’ll closing for quite a few months. Serve chilly or at room temperature.