In a large bowl, combine flour, sugar, yeast, and salt. Gradually add warm milk and knead into a soft dough.
On a floured surface, roll the dough into a rectangle. Spread thin slices of cold butter over two-thirds of the dough. Fold the unbuttered third over the middle third, and then fold the remaining third on top. Wrap in plastic and refrigerate for 30 minutes.
Roll out the dough again into a rectangle and repeat the folding process. Chill for another 30 minutes.
Repeat the rolling and folding process two more times, chilling the dough for 30 minutes after each fold.
Preheat the oven to 400°F (200°C).
Cut the dough into triangles and roll each triangle from the base to the tip to form a croissant shape. Place on a baking sheet.
Brush each croissant with beaten egg for a golden finish.
Bake for 15-20 minutes or until golden brown.