In a small bowl sprinkle the yeast on top of the water, let it sit for about 5 minutes, stir it together, then add the flour and form a smooth dough. Place the dough back in the bowl and let it rise in a not too warm draft free area for about 1-2 hours or doubled in bulk.
In the stand up mixer add the biga, flour, sugar and half the beaten egg. Knead with the dough hook on medium speed until almost combined about 1 minute, then add the remaining beaten egg, knead on medium low speed (#2) for 30 minutes. Move the dough to a lightly floured flat surface and knead and fold about 4 times.Form the dough into a ball, place in a clean bowl, cover the bowl, place it in a not too warm draft free area and let it rise 2 1/2 - 3 hours, until doubled in bulk.
In the stand up mixer add the first dough, the flour, sugar, honey, egg yolk, zest, vanilla and salt, start to knead on low medium speed (#2) for 1 minute, then add the egg one at a time, then continue to knead for 30 minutes, the dough should pull away from the sides of the bowl.
Add the butter a little at a time, kneading continuously, when all the butter has been incorporated, continue to knead for 30 minutes, the dough will be shiny and a little sticky but not overly. Lightly butter your hands and a flat surface, place the dough on top and fold 4-5 times, form into a dough ball, place in the prepared pan seam side up, cover and let rise in a draft free not too warm area for 4-5 hours or until tripled in bulk and the dough is about 1/2 inch over the top of the pan.
Pre-heat oven to 300F (150C).
Once the dough has risen tripled in volume, it should have risen over the top of the pan, bake for about 35-45 minutes or a toothpick comes out clean. If the top is browning too much after 25 minutes then cover with foil and continue baking. Let it sit 30 minutes in the pan then remove and place on a wire rack to cool. Dust with powdered sugar before serving. Enjoy!
The best temperature for this dough to rise is 68-72 F (20-22 C). You want a slow rise.