1ts sugar and yeast into warm milk. Combine hard wheat flour, sugar and salt. Combine with milk mixture by spatula. Make it a lump. Add and mix the room temperature of unsalted butter. After mixing with butter, knead the dough on the worktable. If thin film is formed when the dough is stretched, finish kneading. Shape the dough and place it in the flour-dusted bowl. After cover the bowl with plastic wrap, poke a hole in the plastic wrap. Let the dough rise until double at warm place (1st rise). If the dough is not stick to your finger when you prick the dough with a finger, it means the dough rises well. Press the dough to expel the gas. Divide the dough into sixteen equal parts. Roll out the dough to release gas from the dough lightly. Cover the bowl with plastic wrap for 15 minutes. Grease the 20cm square pan with vegetable oil. After roll out the dough lightly again, place the dough at intervals. Let the dough 2nd rise under the hot water until a half or double. Boil the potato and egg. Peel and mash the potato. Chop the egg fine and mix it the with mashed potato. Combine salt, pepper and mayonnaise. After 2nd rise is completed, dredge dusting flour on the top. After the bread cool down, cut in half. Spread the potato salad on the bread.