In a medium bowl, combine the yeast, ¼ cup of the sugar, and the water. Stir in ½ cup of the flour and cover the bowl. Let rest until doubled, about 1 hour, to make a sponge mixture.
In a large bowl, combine the eggs, the remaining ¼ cup sugar, the orange zest, and seeds. Stir lightly. Beat in the melted butter and the oil. Set aside and let rest while the sponge rises.
In a heavy-duty mixer fitted with a paddle, combine the sponge mixture, egg mixture, and 1 ½ cups of the flour; beat for 3 minutes. Change to the dough hook and add the remaining 2 cups flour and the salt. Knead for 5 minutes. Place the dough in an oiled bowl, turn the dough to coat it, and cover with a damp cloth. Let rise in a warm place until doubled, about 2 hours. Divide the dough into 4 round or cylindrical loaves. Place them on baking sheets lined with lightly floured parchment paper. Cover with a damp towel or plastic wrap and let rise in a warm place until doubled, about 1 hour.
Preheat the oven to 350 degrees F. Slash the top of each loaf in a few places. Glaze the loaves with the egg yolk mixture and dust with confectioners’ sugar. Bake until the loaves are golden and sound hollow when tapped on the bottom, 30 to 40 minutes. Let cool on wire racks.