1. Combine yeast with warm water and sugar in bowl. Let stand 10 minutes. Combine 4 cups of the flour and salt in large mixer bowl. With mixer at low speed, stir in yeast mixture, then 2 whole eggs, and shortening. Mix until blended, 1 minute. On floured surface, knead dough until smooth and elastic, 8 minutes, using remaining 2 cups flour as needed. Place in greased bowl, turning dough to grease top. Cover and let rise in warm, draft-free place until doubled in bulk, 1 hour.
2. Heat oven to 375 degree F. Sprinkle cookie sheet with cornmeal. Punch dough down and knead briefly on lightly floured surface. Divide dough into 6 equal pieces. Roll each into 12-inch-long rope. Place 3 ropes side by side on cookie sheet and braid together; tuck ends under. Repeat, braiding remaining three ropes. Beat egg yolk with 1 tablespoon water in cup; brush over tops of loaves. Sprinkle with sesame or poppy seeds. Cover and let rise 30 minutes.
3. Bake loaves 25 to 30 minutes or until golden. Cool on wire rack. Makes 2 loaves, twenty-four 1/2-inch slices each.