Preheat the oven to 160C/320F and grease 2 x 24 mini muffin pans with butter or cooking spray. Set aside.
In a large mixing bowl melt 350g of the chocolate chips together with the butter (put the other 100g of chocolate chips aside for later). I do this in the microwave - I place both ingredients in a glass bowl and do 30 second bursts until they are melted. Stir together until smooth and glossy and then set aside. Now, add the eggs and egg yolk, vanilla extract and brown and white sugars to a batter jug and whisk together until they are smooth and glossy (like runny caramel). Set this aside.
Now, grab the bowl of melted chocolate and butter. On top of this add the cocoa, flour, sourdough starter, baking soda, salt and egg and sugar mixture, as well as the extra 100g of semi-sweet chocolate chips that you reserved.
Using a spatula, fold it all together until it's thick and glossy. The mixture will be marshmallowy because of the sourdough starter.
Divide the mixture evenly into the 2 x 24 mini muffin pans. I find it easiest (and fairly mess free) to use a disposable piping bag, but a cookie scoop or spoon works too. Fill them around half to ¾ full.
Place the sourdough chocolate brownie bites into the oven for 15 minutes. The tops should be baked but the inside will still be gooey. They will firm up as they cool, so don't be tempted to bake them for too long past 15 minutes.
Allow the bites to cool in the pan for around 10 minutes before you try to remove them, as they will fall apart. As the brownie bites cool, the tops should become crackly and the edges will be a little chewy too. Brownie perfection!
Top with whipped cream or dust with icing sugar before enjoying! I dare you to stop at eating just one!