Dissolve the yeast in the water and flour in a small bowl to make the starter. Rest the starter in a warm place until bubbling (approximately 40 minutes).
Add the dough ingredients, lightly bring together and scoop onto a lightly floured worktop. It is important to have cool hands and work the dough quickly to keep the butter cold.
First turn: roll out the dough to 1cm thickness, fold into thirds, then roll out again. Fold into thirds again, then wrap in cling wrap and chill for 30 minutes.
Second turn: give the dough a quarter turn, roll out to a rectangle, fold into thirds again, wrap, and chill as above. Third and final turn: roll the dough out in the opposite direction from the previous turn to a rectangle, and fold as before. Wrap in cling wrap and chill for at least 30 minutes.
To make your croissants you will need a triangular template measuring 12cm across the base and 24cm high. Roll out the dough into a 24cm x 50cm rectangle, 3mm thick. Trim the four sides of the dough. Using the template as a guide, cut the dough into triangles.
Lay a dough triangle with the base towards you and the smoothest side downwards. Use a knife to make a 1cm-deep incision in the middle of the triangle base, and pull the 2 points of the base slightly to increase the base width and begin the curve.
Roll up the triangle from the base to the point, to form a slight crescent shape. Immediately place on a baking tray. Repeat to make the other croissants as quickly as possible. Put the baking trays in a warm, slightly humid place and let the croissants rise until they have doubled in size, approximately 40 minutes.
Towards the end of the rising time, preheat the oven to 170°C. Lightly brush the croissants with the egg wash and bake for 12—14 minutes. Serve warm with home-made jam if desired.