Remove the dough and butter from the fridge. Roll out the dough on a lightly floured surface until it has 1.5 times the length of the butter and leave 2-3cm on each side and bottom. Try to keep it as rectangular as possible as you roll it out.
Place the butter on the bottom part of the dough, remembering to leave 3cm at the base.
The top third of the dough that doesn't have butter gets folded onto the butter so that now you only see half the butter.
Now the bottom third gets folded over so that you have 3 layers of dough and 3 layers of butter. If your butter isn't too soft at this point, we can go for the first turn. If it has softened, chill the dough for 30-60min wrapped in film before we continue.
Roll the dough on a lightly floured surface, always trying to keep the shape as rectangular as possible. It should have 60cm in length. For the first fold or turn, we're doing a book turn. Use a brush to remove the excess flour from the surface. Fold both edges to the centre so that they meet there and then fold the dough in half so that you have 4 new layers.
Refrigerate the dough wrapped in film for 30min or up to 1 hour if it's a hot day.
Roll out the dough once again, keeping the sides that weren't folded at the top and bottom. Brush off the excess flour and fold in 3 like we did with the inclusion.
Wrap the dough again in film (you can reuse the same strip every time) and refrigerate for 30 minutes or overnight. Make sure that it's tightly wrapped as it will inflate overnight.
After this time, roll out the dough on a lightly floured surface until it measures 30x40cm/12x16in. It will be about 5-7mm thick. Always make sure that it's not sticking to the surface and try to have it as rectangular as possible.
With a knife, trim the edges to have more of a perfect rectangle. Don't through them out! They're still yummy! I roll them and place them in a muffin tin.
Use a ruler to mark every 10cm at each side of the long side of the rectangle. Use a knife to cut the triangles, going from one 10cm mark to the mark across and then one mark over to form the triangle. Repeat with all triangles.
Stretch the base of the triangle and also the length of it so that it's about 3cm/1in longer. Remove the excess flour with a brush. Roll the croissants starting at the base, pulling on the triangle as you go to stretch it further. This way you will have more layers to your croissant. At the end, press down a bit to stick the end to the croissant and leave the tip under it.
Place all the croissants on an oven tray with baking paper or a silicone mat. Let them proof for 2 hours in a warm place.
Paint the proofed croissants with the egg mixed with the water and bake in a preheated oven at 200°C / 390°F for 25-30 minutes. They should be nice and golden.