In large mixing bowl, stir together 2 cups of the flour and the yeast.
In 4-cup glass measure combine milk, butter, sugar, salt, and saffron.
Heat at HIGH for 2 minutes, stirring to dissolve sugar.
Cool to lukewarm (115°F).
Add to mixing bowl with eggs.
Beat 30 seconds on low speed of electric mixer scraping sides of bowl constantly.
Beat 3 minutes at high speed.
By hand stir in enough of the remaining flour to make a soft dough.
Turn out onto lightly floured surface; knead till smooth and elastic, 8 to 10 minutes.
Shape into ball.
Place in greased bowl, turning once to grease surface.
Meanwhile, fill a 4-cup glass measure with 3 cups water.
Heat at HIGH for 7 minutes or till boiling.
Place bowl of dough, covered with waxed paper, in microwave with measure of water.
Heat at LOW (1) for 15 minutes or till dough is double.
Punch dough down; divide in fourths.
Cover; let rest 10 minutes.
Roll 3 portions of the dough to 18-inch ropes.
Loosely braid ropes, starting at center and working out.
Place on greased glass pizza plate.
Divide remaining portion of dough into thirds; roll into three 18-inch ropes.
Loosely braid ropes; place atop larger braid.
Tuck ends under to secure.
Heat 3 cups water in 4-cup glass measure at HIGH for 7 minutes or till boiling.
Place braid, covered with waxed paper, in microwave oven with measure of water.
Heat at LOW (1) for 7 to 8 minutes or till nearly double.
Combine egg yolk and water; brush over braid.
Sprinkle with poppy seed.
Bake in conventional oven at 375°F. for 40 to 45 minutes, covering loosely with foil after first 20 minutes.