Place levain, milk and sugar into Thermomix bowl and blend 5 sec/speed 6.
Add flour and salt and mix 6 sec/speed 6. Knead 3 min/Interval/dough setting.
Turn out onto the bread mat and roll into a tight ball. Wrap and allow to prove for 24 hours in or out of the fridge depending on how cold it is.
The next day, remove dough from the fridge and do the 4 pull and fold as per other sourdough recipes. (see video)
Divide the dough into 4 equal pieces and work with one piece at a time. Line a baking tray with paper or silicone baking mat.
Roll a piece of dough out into a large rectangle, making sure the dough is no thinner than 1/2cm. Take 1/4 of the butter with your fingertips and spread it evenly across the surface of the dough until it is well coated. Roll the dough into a long sausage shape, starting with the short end. You will have a large fat roll of dough. Place it onto the tray and repeat with the remaining pieces of dough. Cover and allow to rise in the fridge for up to 1 hour.
Take each piece of dough and roll flat, by pressing gently onto the dough until it flattens. Use a well floured rolling pin and surface for this without going overboard. When the dough is flat and again in a rectangle shape, turn the dough so the the long side of dough is closest to you.
Place the chocolate into the Thermomix bowl and chop 4 sec/speed 5.
Put a little of the chocolate along the long edge of the dough. Roll the dough away from you, adding more chocolate into the 'roll' as you go. Most of it should end up in the centre of the croissant once cooked, so start with more and end with less as you roll.
Return this roll to the lined tray and repeat with the other three rolls. Cover and prove in a cool environment for 30 minutes.
Cut each roll into 4 lengths, across the roll, so you see the lamination and the chocolate in the centre of the roll. Brush well with egg wash and cover and rise until you are happy with the look of them. If butter begins to melt out of the croissant at any time, they are getting too warm, and you need to put them into the fridge. (It is never too late, baking them straight away will mean the butter will melt even more but the dough soaks it back up once they are cooked and cooled on the tray.)
Preheat oven to 200°C and bake 20 minutes until golden and puffed. Eat hot, or warm. That crunch though!