Cut butter into 1/2-inch-thick slices. In a medium bowl stir slices into 3 cups of the flour until slices are coated and separated. Chill butter mixture while preparing the dough.
For dough, in a large mixing bowl combine remaining 1-1/2 cups flour and the yeast; set aside. In a medium saucepan heat and stir milk, sugar, and salt just until warm (120 degree F to 130 degree F). Add to flour-yeast mixture. Add egg. Beat with an electric mixer on low to medium speed 30 seconds, scraping sides of bowl. Beat on high speed 3 minutes.
Using a wooden spoon, stir chilled flour-butter mixture into dough until flour is moistened (butter will remain in large pieces).
Sprinkle a pastry cloth or surface with 1/4 cup of the remaining flour. Turn dough out onto surface. With floured hands, gently knead dough for 8 strokes. Using a well-floured rolling pin, roll dough into a 21x12-inch rectangle (if necessary, sprinkle surface of dough with enough remaining flour to prevent sticking). Fold dough crosswise into thirds to form a 7x12-inch rectangle. Loosely wrap in plastic wrap; chill 1 to 1-1/2 hours in refrigerator or 20 to 30 minutes in freezer or until firm but not excessively stiff.
On a well-floured surface roll dough into a 2-1/2-inch rectangle. Fold dough crosswise into thirds again and give dough a quarter-turn. Then roll, fold, and turn twice more, flouring surface as needed (it is not necessary to chill dough between each rolling). Place dough in a plastic bag. Seal bag, leaving room for dough to expand. Chill dough for 4 to 24 hours. Use as directed in recipes for Baked Croissants and Filled Croissants (see Recipe Center).