Cut each stick of chilled butter lengthwise into 3 equal pieces.
Place 2 pieces side by side between sheets of waxed paper.
Flatten butter into an 8-inch square using a rolling pin.
Repeat with remaining butter.
Chill butter at least 1 1/2 hours.
Scald 2/3 cup milk; cool to 105° to 115°.
Dissolve yeast in warm water in a large bowl.
Stir in scalded milk, 1/4 cup butter, sugar, salt, eggs, and 2 cups flour.
Beat until smooth.
Gradually stir in enough of the remaining flour to form soft dough.
Turn dough out onto a floured surface, and knead 5 to 8 minutes until smooth and elastic.
Place in a well-greased bowl, turning to grease top.
Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in bulk.
Punch dough down.
Cover and chill 1 hour.
Punch dough down, and turn out onto a lightly floured surface; roll into a 25- x 10-inch rectangle.
Place 1 square of firm butter on center of dough.
Fold dough over butter to make 3 layers.
Turn dough one quarter turn, and roll out.
Repeat twice, placing butter square on center each time.
Cut dough cross wise into halves, and cover and refrigerate for 1 hour.
Remove half of dough from refrigerator; roll into a 12- x 8-inch rectangle.
Cut in half lengthwise; cut each half crosswise into 3 squares.
Cut each square diagonally into 2 triangles.
Roll up each triangle tightly, beginning at wide end.
Seal points, and place point side down on greased baking sheets; curve into crescent shapes.
Repeat with remaining dough.
Refrigerate 30 minutes.
Combine egg yolk and 1 tablespoon milk; brush over croissants.
Bake at 475° for 5 minutes; decrease oven temperature to 400°.
Bake 8 to 10 minutes or until golden brown.