Prepare the Tangzhong: Cook bread flour and water into a thick paste and let it cool.
Knead the Dough: Combine flour, yeast, milk powder, sugar, and salt. Mix in the egg, milk, and cooled tangzhong. Knead in butter until smooth and elastic.
Let It Rise: Place dough in a warm area to double in size.
Make the Garlic Butter Spread: Mix softened butter with minced garlic, salt, and chopped green onions.
Shape and Fill the Dough: Roll out the dough, spread the garlic butter filling, and sprinkle with green onions. Roll and braid the dough, then place it in a loaf pan.
Final Rise and Bake: Let the dough rise again, brush with egg wash, and bake at 350°F (180°C) for 25-30 minutes.