Preheat the oven to 160°C (fan oven).
Melt the butter in a pan over a low heat then add the chocolate bar, half the chocolate chips, vanilla extract and cocoa powder. Heat and stir until fully melted and smooth. Set aside to cool for now.
In a separate bowl, beat the eggs, caster sugar, brown sugar, and groundnut oil for around 5 minutes. You can do this by hand but it’s much quicker and easier with a mixer. Hand mixers work fine.
Add in the sourdough starter discard, and all of the recently cooled chocolate mix from step one. Mix thoroughly together until combined.
In a small bowl, weigh out your flour and then add in the salt and baking powder. Mix through the remainder of the milk and white chocolate chips.
Add the flour mix to the other bowl with your brownie mixture and stir with a spatula. Don’t over mix it, just go until it’s come together and there are no lumps of flour visible.
Line a square baking tin with parchment paper and pour in the brownie mixture. Use the spatula to smooth over the surface.
Bake in the centre of the oven for 40 minutes (keep an eye on it as every oven is different. When it has the matte finish on top and is beginning to crack on the edges, take it out)
Let the baking tray sit until cooled, the brownie is still cooking in it. Don’t be tempted to cut in early!
Once cooled, carefully remove the brownie from the tray and cut into 16 squares, serve up and enjoy.