Boil in a saucepan 220gr of water with sugar, turn off and let cool. In another saucepan, melt 15 grams of butter and when it is still hot add the grated orange peel and the seeds of the vanilla bean. In the remaining 20g of water dissolve the salt. In the mixer break up and dissolve the sourdough starter with 75 grams of syrup of sugar and water and stir until you will have a cream. Gradually add all ingredients (also the remaining syrup) and knead unitil the dought is all on the hook. Wrap the dough in plastic wrap and refrigerate for at least four hours (even until next morning, if the work has been done in the evening).

At this point let's prepare the butter: pull it out of the fridge and put it between two sheets of parchment paper  and with a rolling pin give some small blows on the butter. Flatten it and with the help of a spatula give a square shape 20x20. Do not touch with your hands butter .... do not be silly asking if you have to take with your feet ... always work the butter through the parchment paper. Close it in plastic wrap and keep it in the refrigerator until you're ready to use it. It's really important to be accurate with the measures.

First fold Let flour the work surface. Roll out the dough until you have a square 30x30. Remove excess with a knife and put them in the fridge wrapped in foil (you ill have some other excellent brioches). Take the butter out of the fridge and place it at the center of the dough (without parchment paper), as in the photo. Close the corners of the square of dough over the square of butter and start to stretch the dough, always in the same direction until a 60x22cm rectangle about 1 cm thick. The rectangle must be precise and exact angles ... if it does not trim the sides always putting leftovers in the film. At this point make a fold to 3, that means, starting from the long side to fold on itself a third of the dough and then close "book" the other third. Close the dough in plastic wrap and place in the refrigerator for an hour.

Remove the dough from the fridge. Lightly flour the work surface and roll out the dough stretching (holding the rib of the "book" on the left), until you get a perfect rectangle 60x22 again (and again if not precise trim the edges always preserving the cuttings). Remaking the fold to three. Place the dough in the fridge for an hour, then repeat the procedure one more time. With the third fold, you will have the dough for the croissants, that if done well will have 27 layers (three in the third is 27). Put in refrigerator for an hour.

Final forming Flour the work surface and the rolling pin and roll out the dough until you have a rectangle 50x40. Cut the rectangle into two by obtaining two 50x20. Form from each rectangle triangles: with a ruler starting from the bottom left to make a mark every 8cm and similarly the pastry on top, taking care to the left to start the signs but not from 8cm but from 4cm. Stretch gently holding the base of each triangle with one hand and the vertex with the other. To obtain the crescent shape, drill a small nicks in the middle of the base of the triangle and enlarge the vertices before proceeding with the winding. Roll up tightly with the triangles but without making them too tight and place in baking pan covered with parchment paper well apart from each other (triple) and with the tip pointing down.

At this point you can follow two paths: freeze the croissants before cook them, on a tray covered with a film (and just store them frozen in a bag when they're frozen) or proceed with the leavening.

Place them on a baking sheet covered with parchment paper well spaced, again because they will triple.

Cover the pan with foil and let rise to 20-22 degrees for the whole night.

Once the croissants have raised, the morning after, turn on the oven to 200 °, brush the croissants with beaten egg with milk, sprinkle with granulated sugar. When the oven has reached temperature and bake until cooked (about 15 minutes).

If the croissants were frozen, the evening before cooking place them on the baking sheet and put it to rise naturally. Then the morning after proceed with baking.