First, prepare tongzhong: Mix the flour with the milk in a saucepan, stir until there are no lumps. Turn on the fire to low and heat up the mixture. Keep stirring until the mixture thickens. Remove the saucepan from fire when it starts to become doughy (Be careful when you see the milk start to thicken, as it will turn doughy very quickly) Place in a bowl and cover with cling wrap. Set it aside to cool down for use later.
Add sieved flour into mixing bowl. Add yeast and sugar on one side of the bowl, put salt and the tongzhong on the opposite side.
Add the purple sweet potato powder next to the tongzhong and in the middle of the flour, add the egg. Warm up the milk to room temperature and then pour the milk directly onto the yeast.
Use the spatula to mix the ingredients together to form the dough
Put some flour on the table or kneading mat. Start kneading the dough. It will be sticky at the beginning and you can add a little flour to the table and on top of the dough while kneading. When the dough starts to form a smooth skin, add the butter into the dough and keep kneading. It would probably take around 5 minutes for the dough to absorb the butter and form a smooth skin. (Don't add any flour into the dough after adding the butter)
Once the dough has formed a smooth skin and is no longer sticky, put in back in the mixing bowl and cover it with a cling wrap and let it rest for 1 hour until it doubles its size.
Once it's doubled in size, take it out and place it on the table. Gently press the dough flat to remove the air inside.
Fold the 2 sides of dough into the middle line. Then fold another 2 sides in the way. Keep the folding until the dough shapes like a bun. Then cover the dough with cling wrap again and let it rest for 15 minutes.
Spread some flour on the table and then out the dough upside down on the table with the smooth skin under. Use a rolling pin to flatten the dough. After the dough elongated to about 10"X 6" inches long, start roll the dough from 1 side to another
Cut the dough into pieces but don't separate them. Try to keep the dough into shape. Grease the baking pan with some melted butter. Put the dough into the pan and let it rest for another 20 minutes
Pre-heat the oven in 180C (350F). Put the dough inside and bake for 18-20 minutes depends on your oven's size. I use those built-in oven so I bake this dough for 19 minutes.
Take the bread out and wait for it to cool down. The bread is perfect, soft inside with a beautiful light purple color
