Dissolve the yeast in lukewarm milk. Soft together flour and salt and add to yeast and milk. Stir until smooth.
Turn out on a lightly floured board and knead until smooth and elastic. Place in a greased bowl and grease top of dough. Let rise in a warm place until double in bulk.
Roll dough out on a lightly floured board into a long strip. Dot top of dough with bits of butter. Fold in thirds. Turn halfway around and roll out into a large strip. Fold into thirds. Wrap in waxed paper and chill well.
Roll out again and fold in thirds. Wrap and chill.
Remove from refrigerator and roll out a fourth time. This time roll dough out as thin as possible. Cut into 4" triangles. Roll each triangle from wide end to the tip, pressing in the tip to seal the roll. Shape into half moons or crescents. Place on a well buttered cookie sheet, cover, and let rise until double in bulk.
Brush tops with beaten egg yolk and bake at 375 degrees 10 to 15 minutes. Makes 18 croissants.