Mix the yeast with the water until yeast has dissolved completely.
Add sugar and salt to the flour and quickly stir.
Pour in the yeast mixture and either knead with your hands for 4-5 minutes until a dough forms or use a kitchen machine and knead for 5 minutes.
Place dough covered in your fridge for at least 8 hours or over night (up to 24 hours is possible).
In the meantime place the butter on a sheet of parchment paper and fold a square around it, leaving around 2 inches away from the butter on all sides.
Turn it around and with a rolling pin flattened the butter until the square is completely filled and the height is even.
Place butter into the fridge for at least 30 minutes.
The dough should have doubled in size by now, place it on a lightly floured surface and roll it into a long rectangle shape, twice the size of the butter square.
Remove all flour from the surface with a soft towel or a brush and place the butter on one half.
Fold the dough over the butter, turn 90 degrees and roll out again to a long rectangle and remove excess flour.
Now comes the first “tour”: fold the dough into thirds by folding one side to the middle of the rectangle and then the other side over the first like a letter.
Wrap in plastic wrap or parchment paper and place in the fridge for 30 minutes .
Make two more tours/rounds with leaving 30 minutes between each round to cool the dough.
After the third round roll your dough one last time, fold it twice and roll it into a 10x21 inch rectangle.
Cut triangles with a length of 4 inches on the shirt side.
Make a small slit into the short side and roll the dough tightly into croissant shapes.
Place on a baking sheet onto a baking pan, cover and let rest for 60-90 minutes until croissants have raised in size visibly.
Preheat your oven to 360°F.
Make egg wash by beating the egg with the milk and spread carefully on the croissants.
Bake for 25-35 minutes until golden brown and put on a cooling rack to cool down.