The day before you want to bake the croissants, place the flour, sugar, salt and yeast in the bowl of a food mixer fitted with a dough hook, or a large mixing bowl. Pour in 350-400ml/12-14fl oz cold water and mix until it comes together as a soft dough.
Turn out the dough onto a lightly floured work surface and knead for a good 8-10 minutes, or until the dough feels smooth, stretchy and elastic. Return to the bowl and set aside to rest.
Sandwich the chilled butter between two sheets of greaseproof paper and roll out to a 30x20cm/12x8in rectangle, about 1cm/½in thick. Chill in the fridge until needed.
Turn the dough out again onto a lightly floured work surface. Roll it into a rectangle 60x30cm/24x12in.
Peel the greaseproof paper from the butter and place the butter centrally on top of the pastry rectangle.
Fold one edge of the pastry over the butter to half-cover it, then do the same with the other edge of the pastry so that the butter is covered.
Fold the rectangle in half lengthways, then give it a quarter-turn and roll out again to a rectangle measuring 60x30cm/24x12in.
Repeat the folding, turning and rolling process, as before, three more times (do not roll out the pastry the final time you fold and turn it). Wrap the pastry in cling film and chill in the fridge overnight.
When the pastry has rested, roll it out to a thickness of 0.5cm/¼in. Cut the pastry into two 50x20cm/20x8in strips, then cut each strip into triangles about 10cm/4in wide at the base.
With the narrow point of one of the pastry triangles facing away from you, use your fingertips to stretch the two points of the triangle nearest to you out sideways so that they bow a little.
Roll the pastry over itself, curling it into a traditional crescent shape, so that the furthest point of the triangle ends up curled over the top of the croissant.
Repeat the process with the remaining pastry triangles to create about 20 croissants.
Place the croissants onto baking trays lined with silicone paper or baking parchment. Brush them all over with the beaten egg, then set aside to prove for 30-45 minutes. (NB: The croissants can be frozen at this point, if desired.)
Preheat the oven to 190C/170C Fan/Gas 5. Bake the croissants for 20-25 minutes, or until risen and golden-brown. Serve warm with jam and butter.