Cream butter with 1/3 cup flour.
Roll mixture between 2 sheets of waxed paper to form a 12x6 inch rectangle.
Chill.
Soften yeast in warm water.
Combine milk, sugar, and salt.
Cool to lukewarm.
Add softened yeast and egg.
Mix well.
Add 3 3/4 cups flour (or enough to make soft dough).
Knead on lightly floured surface till smooth and glossy, about 5 minutes.
On floured surface, roll dough in 14 inch square.
Place chilled butter on half the dough.
Fold over other half of dough, sealing edges well.
Roll to 20x12 inch rectangle.
Fold in thirds so you have 3 layers. (If butter softens, chill after each rolling.)
Roll to 20xl2 inches again.
Fold and roll twice more.
Then fold in thirds to form 12x7 inch rectangle.
Chill 30 minutes.
Cut dough crosswise in fourths.
Roll each fourth (keep remainder chilled) in 22x7 inch rectangle, 1/8 inch thick.
Cut in 10 pie shaped wedges, 4 inches at base and 7 inches long (you will have an extra 1/2 wedge on each end; put together, if desired).
To shape Croissants: Begin with base (if dough has shrunk back, pull to original size) and roll loosely toward point.
Place 3 inches apart on ungreased baking sheet with center point down; curve ends to form crescents.
Cover and let rise till almost double, about 1 hour.
Beat egg yolk with milk; brush on rolls.
Bake at 375° for 12 to 15 minutes or till lightly browned.