In a small bowl, sprinkle yeast over the lukewarm milk and sugar. Mix together and allow to sit until frothy, about 10 minutes.
In the bowl of a stand mixer fitted with a dough hook or a large bowl, combine the bread flour and salt.
Mix in the Tangzhong mixture and yeast with milk and sugar. Slowly mix in pieces of the butter until the dough completely comes together. It should be smooth and slightly tacky. If it is too dry, add a little more water. If it is too sticky to handle, add a little more flour.
Grease a large bowl with butter or oil and add the dough, turning to coat. Cover and allow to rest at room temperature in a warm place until doubled, about 1 hour.
On a lightly floured surface, punch down the dough and divide into 2-3 equal pieces. Form each into a smooth ball, cover, and allow to rest for 20 minutes.
Grease a loaf pan with butter.
On a lightly floured surface, roll one ball into a thin rectangle or ellipse. Fold the dough up lengthwise in thirds and pinch together the ends to seal. Roll the dough up lengthwise, sealing the seam, and place in the loaf pan with the spiraled edges facing the long sides of the pan and the seam facing down. Repeat with remaining dough. Cover and allow to rest until the dough puffs up and fills in the pan, 40 minutes to 1 hour.
Preheat oven to 350˚F.
Bake the dough in the preheated oven until golden brown with a solid crust, about 35 minutes. Remove from the pan to a wire rack to cool to room temperature before serving.