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.
Turn out onto Silpat mat and roll into a tight ball. Wrap and allow to prove for up to 18 hours in the fridge.
Meanwhile, roll butter between two pieces of baking paper into a large flat square about 1cm thick. Refrigerate.
The next day, remove dough from the fridge and do the 4 pull and fold as per other sourdough recipes. (see video)
Roll dough out into a square about 2cm thick. It needs to be slightly larger than the prepared butter block. Place butter into centre of dough and fold corners in to meet in the middle as though it were an envelope.
Roll dough out again. Fold bottom third up and top third down over bottom. Roll flat and wrap. Refrigerate for 30 minutes.
Do this three more times. The dough and butter will start forming thin layers, this is lamination. (See tutorial video on croissant techniques)
Preheat oven to 200ºC and line a large baking tray with paper.
Roll dough out one final time to a large rectangle around 2cm thick.
Cut into triangles and roll, starting with wide part and finishing with pointy end. Allow to rise again, covered in a draft free environment, until you get the wobble without over warming! Brush with egg wash and bake for 15 minutes or until golden and puffy. Eat immediately!