Dissolve yeast in warm water.
Stir in sugar, shortening, 1 egg, the salt and 3 1/2 cups of the flour.
Beat until smooth.
Mix in enough remaining flour to make dough easy to handle.
Place dough in greased bowl; turn greased side up.
Cover; refrigerate at least 2 hours.
When dough rises, punch it down.
Cut off amount of dough needed and return remaining dough to refrigerator.
Dough can be kept 3 to 4 days.
About 2 hours before baking, shape bits of dough into 1- to 1 1/4-inch balls.
Blend egg yolk and 2 tablespoons water; brush tops of balls with mixture and dip into poppy seed.
Place 1 inch apart on lightly greased baking sheet.
Cover; let rise until double, 1 to 1 1/2 hours.
Heat oven to 400°.
Bake 10 minutes or until golden brown.