In a small bowl, sprinkle yeast over the lukewarm milk. Mix together and allow to sit at room temperature 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, sugar, and salt.
Mix in the Tangzhong mixture and yeast with milk. 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 lukewarm milk. 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 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. Fold the dough lengthwise into thirds and pinch together the ends to seal.
Cover and allow to rest until the dough puffs up and fills in the pan, about 1 hour.
Preheat oven to 350˚F (180˚C).
Bake the dough in the preheated oven until golden brown with a solid crust, about 40-45 minutes.
Remove from the pan to a wire rack to cool to room temperature before serving.