In a large bowl, dissolve yeast in warm milk. Add the butter, sugar, salt and egg yolk; mix well. Stir in enough flour to form a soft dough. , Turn onto a floured surface; knead until smooth and elastic, 6-8 minutes. Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 1 hour. , Punch dough down. Shape into 12 balls. Push thumb through centers to form a 1-1/2-in. hole. Stretch and shape dough to form an even ring. Place on a floured surface. Cover and let rest for 10 minutes; flatten bagels slightly. , Fill a Dutch oven two-thirds full with water; bring to a boil. Drop bagels, 2 at a time, into boiling water. Cook for 45 seconds; turn and cook 45 seconds longer. Remove with a slotted spoon; drain well on paper towels. , Sprinkle with sesame or poppy seeds if desired. Place 2 in. apart on greased baking sheets. Bake at 400° until golden brown, 20-25 minutes. Remove from pans to wire racks to cool.