Cook in medium low heat, stirring continuously with a whisk to prevent burning. The mixture will become thick. If you are able to see some lines for every stir you make with the spoon, your tangzhong is ready. Remove it from the heat.
Transfer it to a clean bowl and cover with a cling wrap sticking on the surface of the tangzhong. Let it cool completely. You can use it right away once it cools down to room temperature.
Take the flours, sugar, salt, milk powder and instant yeast in a bowl and make a well in the centre.
Take the milk, mashed potatoes and tangzhong in another bowl, whisk well and add to the dry ingredients.
Knead everything as a dough, add the butter and knead again for a while. The dough will be smooth but not sticky and elastic.
To test, stretch the dough. If it forms a thin membrane, your dough is done. Knead it as a ball, place in a greased bowl, cover and let it sit in a warm place for about an hour.
Transfer the dough to a flat surface, divide into three or four equal parts, place it in a plate and let it sit again for half an hour covered with plastic.
Roll each ball to form a log like loaf, place in a greased loaf pan and keep again for an hour to double the volume.
Preheat the oven to 350 degrees F.
Brush the loaf with milk and bake for 25-30 minutes until the crust turns golden brown.
Remove the bread from the pan and cool it completely before slicing. Recipe courtesy: Priya Easy N Tasty Recipe