Pour the boiling milk into a mixing bowl, stir in the butter and the sugar and leave mixture until lukewarm. Sprinkle over the crumbled yeast en let stand for 5 minutes (if using fresh yeast) or 15 minutes (if using dried yeast). Stir in the salt en egg white, then gradually beat in the flour to make a soft dough. Knead the dough on a lightly floured surface for about 5 minutes, until smooth and elastic. Retorn the dougn to mixing bowl, cover with cling wrap and leave to rise in a warm place for about 1 hour. When doubled in size, knock out the air and divide the dough in 16 even-sized pieces. Oll each piece into a sausage shape, then form a ring; pinch the join firmly together. Place bagels on greased baking sheets, leaving enough space between them to let them rise and swell without touching each other. Cover loosely with a towel and leave them to rise for 10-15 minutes. Bring water to the boil in a large saucepan, reduce the heat until waters simmers, then gently drop in the bagels - only a few at a time to prevent them from touching and sticking together. Simmer each bagel for 10-15 seconds or until puffy and beginning to swell. Lift the bagels out of the water with a slooted spoon and return them to the baking sheets. Mix the egg yolks with the cold milk and brush the bagels with this mixture. Bake bagels for about 20 minutes in preheated (200 Centigrade/400 Fahrenheit) until golden and hollow sounding when tapped. Leave them to cool on a wire rack.