Measure yeast into large mixing bowl. Cool scalded milk slightly, add cold water, and then add to yeast and sugar. Add salt, egg yolks, and butter, then add half the flour; beat until smooth. Add remaining flour a little at a time until you have soft dough that can be handled. Turn onto floured board and knead a little. Leave dough in a warm place, free from drafts, until doubled. Shape into a long roll about 2 inches in diameter, and cut off pieces to half fill tart shell. Or cut chunks of dough and work smooth with your hands to half fill the shells. With your finger, make a dent in the center; moisten this hole with a little egg white (to make it stick), and press in a small round ball of dough. Let rise until very light, then brush with beaten egg white. Bake until well browned, about 20 minutes.