In a bowl, combine flour, yeast, salt, and sugar.
Add the tangzhong, milk, and egg to the dry ingredients.
Knead until a shaggy dough is formed, about 5 minutes if using a stand mixer on low speed.
Cover the bowl and let the dough rest for 15 minutes.
After the initial resting, add the butter gradually while kneading slowly. Knead the dough for 15 minutes or until the dough is soft and elastic.
Form the dough into a ball and transfer it into an oiled bowl. Cover and let it rest for 1-2 hours or until the dough has doubled in size.