In the microwave, melt the chocolate and butter/spread. Stir it often so it does not burn.
Whisk in the sugar, salt, and vanilla.
Add the eggs one at a time, whisking to combine each addition.
Sift the cocoa powder over the chocolate and stir to combine.
Add the starter and stir gently until it is completely incorporated and you don't see any streaks of starter.
Place the batter into a 20x20 cm (9x9 inches) square baking pan lined with baking paper.
Bake in a pre-heated oven at 165°C – 325°F for about 30-40 minutes, or until a toothpick inserted into the center comes out clean.
Cool in the pan for 20 minutes, then lift the baking paper out and allow to cool the rest of the way on a wire rack.
When completely cold, cut into 12 pieces, and enjoy!
Recipe Notes
Make sure you do not overbake these brownies!
For more cake-like brownies, use the full amount of sourdough discard. For denser, chewier brownies, user about 65 grams – ¼ cup instead of the full amount.