To make the poolish combine the flour and yeast in a medium bowl and mix with your fingers. Pour in the water and mix until thoroughly combined. The mixture should have the consistency of a thin pancake batter.
Cover the bowl loosely with cling film and let it sit at room temperature for 12-15 hours. The mixture will be bubbly and ready for use.
Make a butter block by combining the butter on a piece of grease proof paper, then place another piece of paper on top and hit the butter with a rolling pin to begin to flatten it. Keep flattening the butter into a rectangle that is 15cm x 20cm.
Wrap it tightly in the grease proof paper and refrigerate it until you are ready to use it.
To make the croissant dough, mix the flour, sugar and yeast in the bowl of a stand mixer fitted with the dough hook and give it a quick mix on the lowest setting to distribute all of the ingredients evenly.
Pour about half the water around the edges of the bowl of poolish to help release it, then add the contents of the bowl along with the water (holding back 50g) to the mixer.
Add the butter and mix on low speed for 2 minutes to moisten the dry ingredients. Scrape down the sides and bottom of the bowl to make sure all the flour has been incorporated.
Sprinkle the salt over the top and mix on low speed for 2 minutes to dissolve the salt. If the mixture feels at all dry add the reserved water in very small amounts as needed. Continue to mix on low speed for 20 minutes.
Run a bowl scraper around the sides and bottom of the bowl to release the dough and turn out onto the work surface. Stretch the left side of the dough outward and fold it over the centre of the dough, then stretch and fold the right side over to the opposite side, as if you were folding a letter.
Repeat the process working from the bottom and then the top. Turn the dough over, lift it with a bench scraper and place the dough back in the mixing bowl. Cover with cling film and leave to sit at room temperature for an hour.
Prepare a baking tray with baking parchment, uncover the dough and use a dough scraper to release the dough and turn it out onto a floured surface. Gently but firmly pat the dough into a rectangle approximately 25cm by 19cm, press any large gas bubbles out of the dough and place the dough on the baking tray and put it in the freezer for 20 minutes.
Lightly flour the work surface and a heavy rolling pin. Turn the dough out onto the work surface and lightly dust the top with flour. Roll the dough outward from the centre, rotating it frequently and flipping and fluffing it from time to time, adding just enough flour to the work surface, dough and or pin to prevent it sticking until you have a rectangle of dough that is around 1cm thick.
Lay the butter block across the centre of the dough, stretch and fold over the two longer sides so they meet in the centre and pinch them together to seal.
Using the rolling pin, press down firmly on the dough across the seam from one side to the other to expand the dough. Turn the dough so a short end faces you. Roll to expand the length of the dough turning the dough over and adding flour only as needed, until you have a rectangle that is 50cm by 22cm and 1cm thick.
Fold the bottom third of the dough up as if you were folding a letter. Fold the top third down to cover the bottom third. Turn the block 90 degrees, so the dough resembles a book with the opening on the right. This completes the first turn, return the dough to the pan, cover with clingfilm and freeze for 20 minutes until the dough is stiffened but not hard.
For the second turn, lightly dust the work surface with flour, place the dough on the surface with the opening on the right. Work quickly with the dough so it doesn't warm up, but be careful not to expose the butter.
Expand the dough by pressing firmly with the rolling pin, working up the length of the dough. If it cracks, stop a minute and let the dough warm a bit more at room temperature. Roll out the dough as before to a rectangle that is 50cm by 22cm and 1cm thick. Repeat the letter fold as before and return the dough to the freezer for 20 minutes.
For the final turn repeat the same as the step above and then place the dough back in the freezer for 20 minutes. Cut the dough crosswise in half, making two rectangles. Cover two baking trays with parchment paper.
Lightly flour the work surface. Remove one piece of dough from the freezer and position it on the work surface with the short end towards you; transfer the second piece of dough (if using at this time) to the fridge.
Roll the dough out to a rectangle 50cm x 20cm. Turn the dough so that a long side is facing you and trim it to a neat rectangle. Trim the remaining sides only as needed for straight edges.
Cut the dough in half, so that you have two squares. Then cut each square in half so that you end up with four rectangles. Cut each rectangle diagonally so that you end up with 8 right triangles.
Hold one triangle up by the base with one hand and, using your fingertips, gently pull the dough until it is stretched to about 30cm.
Put the dough on the work surface, with the base of the triangle close to you. cut a snip in the middle of the base about 1.5cm long, roll the dough up from the wide end to the tip. Put on a prepared sheet with the tail tucked under. Repeat with the remaining 7 triangles of dough, spacing them evenly on the baking tray.
If baking all the croissants now, repeat these steps with the remaining dough in the fridge until you have 16 croissants.
Brush the croissants with the egg wash, cover the pans with plastic tubs or cardboard boxes and let prove for about 2 hours.
Position the racks in the upper and lower two-thirds of the oven. Preheat to 170C Fan/190C/350°F.
Brush the croissants again with egg wash. Bake for 35-40 minutes, rotating the pans once halfway through baking and separating the croissants if they are touching, until the tops are a rich golden brown and no portions, particularly between the layers, look under cooked. Set the pans on a rack and cool completely.