Dissolve the sugar in water and then mix with other ingredients, and knead into dough. Leave it in a covered container at room temperature. When it doubles, add:
100g Manitoba flour
25g brown unrefined sugar dissolved in 45g water
And repeat the procedure mentioned above. When it doubles, use it in the final croissants dough. This whole procedure for sweet starter takes about 1.5 day, depending on the room temperature and the strength of your starter.
The sweet sourdough starter is not my invention, I learnt it at the Quest for sourdough event in Belgium last year, where there's the very first sourdough library in the world. It was introduced to me by Stefan Cappelle. See here: http://www.questforsourdough.com/sourdough/sweetsourdough
100% sourdough croissants dough
Mix all the ingredients (except butter) into nice and smooth rather stiff dough, refrigerate in a container with a lid overnight.
Next day leave the dough at room temperature for about 45 minutes (my kitchen is 21 °C).
In the meanwhile prepare the butter, beat it and roll to a thickness of 0.5 cm in a 21-cm square. It should be mouldable, if it's too soft, freeze it for 3 minutes. The dough and the butter should be the same temperature, not too soft and not too stiff. Roll the dough into a square bigger than the butter one, so you can wrap it in (see internet videos on how to lock in the butter square).
Do 3 series of letter folds. Between each fold put the laminated dough in the fridge for about 30 minutes (wrap it in cling foil). If the dough resists, let it rest for about 10 minutes and then roll it. Always roll from the centre in each direction. Don’t use too much flour, brush off excess.
After the 3rd fold, wrap the dough in cling foil and refrigerate overnight. Next morning take it out, let it warm up (about 30-45 minutes), and roll it about 0.4cm thick, cut and roll to croissants.
Brush with a mixture of 1 egg, half tea spoon of cream and a pinch of salt. Cover the pan with cling foil; it shouldn't stick to the croissants. Proof till puffy and about 2 times (2.5x) their original size. At 21 degrees Celsius it took about 8-9 hours.
Brush again with the leftover egg-cream-salt mixture.
Preheat the oven to 220 °C; bake first 5 minutes at this temperature (thank you very much for this advice, dear Ralph @breadworks_2)
After that lower to 190 °C and bake until golden brown (in my oven it takes 25min but my oven is 15-years old). Leave to cool if you can wait, best served a bit warm.
Enjoy your day and let's bake the world a better place!
You can find me on Instagram under sourdough_mania :)