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.
Mastic gum, also called mastiha, is the dried resin of a Mediterranean tree. Grind it on a microplane zester or in a spice grinder. Christopsomo can be stored, wrapped in plastic, at room temperature for up to 3 days.