Preheat the oven to 356°F (180 °C). Prepare a flat oven dish, covered with baking paper. Use a mixer with dough beaters.
Pour the flour, dry yeast and salt in the mixer's bowl. Turn it on low to mix the products. Gradually, add the warm milk while stirring nonstop. Form a ball of dough, take it out of the bowl, knead lightly and cover it for 20-30 min. to relax. Roll it out to a rectangular sheet about 1/4″ (0.5 cm) thick using a rolling pin.
Divide the butter into 2 parts. Smear the sheet with part of 1 half of the butter and fold it in 2. Smear it again and fold once more. Repeat until you've formed a cube. Wrap it with foil and leave in the freezer for 30 min. to cool.
Roll out the cooled dough into a rectangle (long strip) once again and smear it with the rest of the butter. Be sure to use your butter wisely so you have enough for each time you fold it until you form a cube. Wrap it in foil and leave in the freezer for 30 min.
Once well cooled, take it out. It's now ready for making croissants. Roll it out into a long, wide strip, while only rolling back and forth. Cut it into equilateral triangles 8″ (20 cm) per side, roll them up into croissants, while carefully pulling on the dough with your hands sideways.
Arrange the formed croissants in the oven dish and leave them to rise about 30-40 min. Smear them with a lightly beaten egg.
Bake at 356°F (180 °C) about 20-25 min.
Note: The amount of flour is quite a lot and it's better to divide the dough into 2 separate, equal parts when working with it.