Mix butter, flour and salt. Add milk, egg yolks, sugar and lemon rind. Add the yeast which has been mixed well with the water and the ½ teaspoon of sugar. Let rise in a warm place until doubled in bulk.
Roll out about 1 inch thick on a well-floured board. Cut in rounds with a large cookie cutter and press on top of each one a pitted prune which has been soaked in a little water until soft. Or make a well on top of each and fill it with a thick marmalade. Brush the tops with lightly beaten egg white and sprinkle with additional sugar. Place 1 inch apart on greased cookie sheets (or in muffin tins) and let rise until doubled in size.
Bake in a preheated hot oven (450°) 20 to 25 minutes.