Combine the flours, yeast, 1/3 cup + 1 tablespoon of sugar, the salt, water, 5 eggs, the white chocolate, and 1 tablespoon of orange zest in the bowl of an electric mixer fitted with the dough hook. Beat on low speed for 2 to 3 minutes, until the dough starts to come together. Beat on low speed for another 3 to 4 minutes, until the dough is stiff and dry.
With the mixer on low speed, add the butter, one piece at a time, mixing after each addition until the butter disappears into the dough. Increase the mixer speed to medium and beat for another 15 minutes, until the dough becomes sticky, soft, and somewhat shiny. Increase the speed to medium-high and beat for another 2 to 3 minutes, until the dough makes a slapping noise against the sides of the bowl. The dough is ready when you can gather it into a ball and pick it up in one piece.
Place the dough in a large bowl and cover with plastic wrap, pressing the wrap directly onto the surface of the dough. Let rest in the refrigerator overnight.
Lightly grease two 9-inch loaf pans. Divide the dough in half and press each piece into a 9-inch square. Fold down the top third of the dough toward you, and then fold up the bottom third, as if folding a letter. Press gently to seal the layers. Place the dough, seam side down, in one of the prepared pans. Repeat with the second piece of dough and place it in the second pan. Cover the loaves with a clean dish towel and let rise in a warm, draft-free area for about 4 hours, until the loaves have doubled in volume.
Preheat the oven to 350°F. Beat the remaining egg in a small bowl. Gently brush the tops of the loaves with the egg wash. Stir together the remaining 3 tablespoons of sugar and 1 tablespoon of orange zest in a small bowl. Sprinkle the orange sugar over the tops of the loaves. Bake for 30 to 45 minutes, until the tops and sides of the loaves are golden brown. Remove from the oven and let cool in the pans for 30 minutes. Invert the loaves onto a wire rack and let cool completely.