1. Dissolve the yeast in the milk.
2. Sift together the flour and salt and stir in the yeast mixture. Knead until smooth and elastic.
3. Place the dough in a greased bowl, grease the top of the dough and let rise in a warm place (85°F) until doubled in bulk.
4. Roll the dough out in a long strip, dot with bits of the butter and fold into thirds. Turn so that an open edge is nearest. Pat and roll into another long strip, fold into thirds, wrap in wax paper and chill well.
5. Roll the chilled dough out and fold the ends to the center. Fold again as above, wrap and refrigerate. Remove and repeat the process a fourth time.
6. The last time, roll the dough out a little thinner (about 1/8 inch) but do not fold. Cut into triangles and brush 1 tip of each with beaten egg yolk mixed with a little water. Roll from the wide end to the tip, pressing to seal. Shape into half-moons.
7. Place on well-buttered cookie sheets, cover with wax paper and let rise until doubled in bulk.
8. Brush the tops with beaten egg yolk and bake in a preheated 425°F oven 20 to 25 minutes. Serve hot or cold.