Pour warm water into a bowl, and sprinkle with 1 envelope of yeast. Stir with a fork until yeast has dissolved, then let stand until foamy, 5 to 10 minutes. Stir in 1/2 cup flour, and cover bowl with plastic wrap. Let stand in a warm place until doubled in bulk, about 30 minutes.
Pour warm milk into a bowl, and sprinkle with remaining envelope of yeast. Stir with a fork until yeast has dissolved, then let stand until foamy, 5 to 10 minutes.
Meanwhile, whisk together sugar, whole eggs, 2 egg yolks, salt, and vanilla. Whisk in milk mixture.
Beat butter and remaining 3 1/2 cups flour with a mixer on medium speed until mixture is crumbly. Reduce speed to low, and gradually add egg mixture. Raise speed to medium, and beat until smooth.
Add yeast-and-flour mixture, and beat on high speed until dough is elastic and long strands form when it's stretched, about 5 minutes. (It will still be very sticky.) Stir in dried fruits and zests. Transfer to a buttered bowl, and cover with buttered plastic wrap. Let dough stand in a warm place until doubled in bulk, about 2 hours.
Preheat oven to 400 degress, with rack in lower third. Turn dough out onto a floured surface, and divide into 4-ounce portions. Knead a few times, then shape into balls. Drop each ball into a paper mold, transfer to a rimmed baking sheet, and loosely cover with buttered plastic wrap. Let rise in a warm place until dough reaches edge of mold, 30 to 45 minutes.
Whisk remaining yolk with cream, and brush onto tops of balls. Cut an X in the top of each ball with kitchen shears.
Bake 10 minutes. Reduce heat to 350 degrees, and bake until tops are golden brown and rise slightly above rims of molds, about 15 minutes. Tent baking sheet with foil if tops are beginning to get too brown. Transfer panettone to a wire rack to cool. Panettone can be stored in an airtight container for up to 3 days.