1 egg yolk beat with a splash of milk for the glaze
In a mixing bowl, activate 10 g yeast with 150 g of tepid milk. Next add 125 g of flour into the mixture, stir well, cover with a damp tea towel and leave to rest for 30 minutes. Then add the rest of the flour, milk, sugar, yeast and salt and knead until you achieve an even dough. Leave in a cool place overnight.
Roll the cold butter into a square between two sheets of greaseproof paper. On a work surface powdered with flour, spread the dough into an even bigger rectangle. Place the butter in the middle and close it in by folding over the edges of the dough. Roll the dough into a large rectangle about 0.5 mm thick.
Fold in three and make a quarter turn, just like for the puff pastry and then place in the refrigerator for 30 minutes. Repeat twice, making sure that the folding is straight. Roll the dough to around 4 mm of thickness, then cut into triangles, each with a 10-12 cm base. In the middle of each base, facing the opposite point of the triangle, make a 1 cm cut. Form the croissants by rolling from the base and then place on a baking tray covered in greaseproof paper.
Leave to rise in a warm place for two hours. Preheat the oven to 220°C (th. 7). Before placing in the oven, cover the croissant with the egg yolk and milk wash and then cook for 15 to 20 minutes. Leave to cool. Now you can eat them!