Dissolve yeast in 1 cup warm water in a large bowl; stir in honey and saffron threads. Let stand for 5 minutes. Add melted butter, 1 teaspoon salt, and egg; stir well with a whisk.
Lightly spoon flour into dry measuring cups; level with a knife. Add 2 3/4 cups flour to yeast mixture, and stir until a soft dough forms. Cover and let stand for 15 minutes.
Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 8 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will be very soft).
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 40 minutes or until doubled in size. (Gently press two fingers into dough. If indentation remains, the dough has risen enough.)
Punch dough down. Shape dough into a ball; return to bowl. Cover and let rise an additional 40 minutes or until doubled in size. Punch dough down; cover and let rest 15 minutes.
Divide dough into 3 equal portions. Working with 1 portion at a time (cover remaining dough to prevent drying), on a lightly floured surface, roll each portion into a 25-inch rope with slightly tapered ends. Place ropes lengthwise on a large baking sheet sprinkled with cornmeal; pinch ends together at untapered ends to seal. Braid ropes; pinch loose ends to seal. Cover and let rise 20 minutes or until almost doubled in size.
Preheat oven to 375°.
Combine 1 teaspoon water and large egg yolk, stirring with a fork until blended. Uncover loaf, and gently brush with egg yolk mixture. Sprinkle evenly with 1/4 teaspoon poppy seeds. Bake at 375° for 30 minutes or until loaf sounds hollow when tapped. Cool on a wire rack.