On a clean work surface, mound 3 of the remaining cups of flour and make a well in the center. In a medium bowl, beat together 4 of the egg yolks, 1/2-cup sugar, the butter and 1/2 cup water. Add the yeast combination from above and mix well. Pour the entire mixture into the flour well and gradually mix the flour into the liquids to form a sticky dough. Knead the dough for 5 to 10 minutes, or use the dough hook attachment on an electric mixer. The dough should remain somewhat tacky, unlike bread dough.
Punch down the dough and add the remaining flour, egg yolks, 1/2 cup water, egg, sugar and lemon zest and knead until blended, then knead for an additional 10 minutes on a floured work surface. Place in a greased or buttered bowl and cover with plastic wrap, then let rise for an additional 2 hours.
In a small bowl, combine the confectioners' sugar and water and stir well. Drizzle the pandoro with the mixture and serve in wedges.