Cut the orange into chunks and chop finely with the sugar in a food processor. Set aside. In a small bowl, add the yeast to the lukewarm water and set aside. Pour the milk into a large bowl; stir in the chopped orange, dissolved yeast, honey, lemon zest, and vanilla. Add 2-1/2 cups of the flour to make a sponge. Beat with a wooden spoon for 100 strokes. Cover and set aside for 15 minutes.
Stir the cardamom, egg, butter, and salt into the sponge. Add enough of the remaining unbleached flour, about 1/3 cup at a time, to make a soft, kneadable dough. Turn the dough out onto a floured surface and knead for 8 minutes, using enough of the remaining flour to keep the dough from sticking. Place the dough into a lightly oiled bowl, turning the dough to coat its surface. Cover the bowl with plastic wrap; place in a warm spot until doubled, 1 to 1-1/2 hours.
Punch the dough down and turn it out onto a floured board. Divide the dough in half; knead and shape each half into a ball. Place the balls onto a large, lightly buttered baking sheet, seam side down, leaving room between them. Cover loosely with plastic wrap and let rise in a warm spot until almost doubled.
Preheat oven to 375 degrees F. Brush the loaves with egg wash and sprinkle with sugar. Bake for 50 minutes. The surfaces will become dark and crusty. Cool loaves on a rack.