Place the flour in the freezer for at least one hour.
Mix the water and milk and stir in the yeast with a mixer. Stir egg, sugar and salt into the mixture. Add flour and stir for 2-3 minutes until you have a firm dough.
Wrap the dough in kitchen wrap and place in the freezer for 15 minutes.
Cut the cold butter into slices and place these in between two sheets of baking paper. Then press the butter into a single 15x15 cm square of uniform thickness.
Place the cold dough on a table sprinkled with flour. Roll it out to 30x30 cm, place the butter in the middle of the dough and fold the ends around the butter so that the butter is completely enclosed in the dough. Put the dough in the refrigerator for 45 minutes and then it is cold and ready to be rolled out.
Sprinkle flour on a table and roll out the dough to 1 cm thickness, measuring about 30x60 cm. Brush the flour off of the dough, fold 1/3 of the dough in towards the middle and the other 1/3 on top, and you now have 3-layer dough. Place the dough in the refrigerator for 30 minutes. Repeat this entire step 3 times. Place the dough in the refrigerator for 30 minutes and then it is ready.
Roll out the dough to approx 25x60 cm. Cut the dough into two pieces measuring 12.5x60 cm. Cut each piece into 6 squares, which are then cut diagonally, resulting in 24 triangles.
Roll up the dough from the wide edge. Place the croissants on a baking sheet and allow them to rise for about 1-2 hours until they have doubled in size.
Bake the croissants in a pre-heated oven at 200 degrees Celsius for about 15 minutes until they are golden brown. Cool the croissants on a cooling rack.
