1 tablespoon plus 1 1/2 teaspoons active dry yeast (two 1/4-ounce envelopes)
8 cups unbleached bread flour, plus more for surface and more if needed
1 3/4 cups sugar
2 cups whole milk, warmed
2 teaspoons ground anise seeds
1 teaspoon coarse salt
1 teaspoon finely grated orange zest
1 teaspoon ground mastic gum
4 ounces (1 stick) unsalted butter, melted, plus more for pans
5 large eggs, lightly beaten
Vegetable oil, for bowl
1 large egg yolk, lightly beaten with 1 tablespoon whole milk, for egg wash
3 tablespoons sesame seeds
Sprinkle yeast, 1 cup flour, and 1/4 cup sugar over milk in a medium bowl. Cover, and let stand for 1 hour.
Combine remaining 7 cups flour and 1 1/2 cups sugar, the anise seeds, salt, orange zest, and mastic gum if desired. Make a well in center of mixture, and add milk mixture, melted butter, and eggs. Mix together ingredients using your hands. Once combined, turn out dough onto a lightly floured surface, and knead, adding more flour if needed, until smooth and supple, about 15 minutes.
Transfer dough to a lightly oiled bowl. Cover with plastic, and let rise in a warm place until doubled in volume, 1 1/2 to 2 hours.
Butter two 9-inch springform pans. Punch down dough, divide in half, and roll each half into a 24-inch-long log. Coil each log into a mounded circle (similar to a snail shell). Transfer each to a prepared pan. Cover with plastic, and let rise until doubled in volume, 1 1/2 to 2 hours.
Preheat oven to 375 degrees. Brush egg wash over tops of loaves, and sprinkle with sesame seeds. Bake until golden brown, about 45 minutes. Transfer pans to a wire rack, and let cool for 15 minutes. Run a knife around edge of pans, and remove bread. Let cool completely on wire rack