Preheat oven to 180°C and butter and line a 20 x 30cm baking tin with paper, with some extending above the edges for easy brownie removal. If you use a smaller tin, you will get a deeper brownie and cooking time may vary slightly.
Place 170g of the chocolate into the Thermomix bowl and mill 10 sec/speed 10. Add butter and melt 7 min/50°C/speed 1. Stop and scrape down sides of bowl about half way through. Add more time if required, but you should have a nice smooth mixture.
Add sugars, cocoa, salt, eggs and vanilla and blend 6 sec/speed 6. Add sourdough discard and blend 6 sec/speed 6.
Add flour and combine 6 sec/speed 6. Scrape down sides of bowl with a spatula and fold in any flour that is not incorporated.
Add half of the remaining chocolate and half the pecans and stir through. Spread into the prepared tin and tap gently to settle the mixture.
Bake 38 minutes. Remove from the oven and top with the remaining pecans, the chopped chocolate (this needs to act like glue to hold the nuts in place) and a few pink salt flakes. Return to the oven and bake further 2-3 minutes until the chocolate has melted.
Cool on a wire rack in the tin until completely cold before removing it and cutting into squares. (However, as warm brownies are almost irresistible, we suggest you cut off one corner and have a taste before they are totally cold!