In a large bowl or the bowl of your mixer, weigh out and add your all-purpose flour, sugar, salt, potato flour, milk powder, instant yeast, and scallions and stir to combine all of your dry ingredients making sure there are no clumps. 

Weigh the rest of your ingredients and add them to the bowl. 

If using a stand mixer, knead for 8 minutes on medium speed. If you are kneading by hand, you will need about 15 minutes of kneading after all the ingredients are incorporated together. 

The dough will be a bit sticky. 

Place your dough in a lightly oiled bowl and cover. Let dough rise in a warm place for 1 to 1.5 hours. The dough should be doubled in size. 

Remove dough from bowl to a lightly oiled counter and divide into 6 pieces. I like to weigh my buns at this point to keep them all fairly consistently sized and I usually get 6 buns that are all between 85 and 90 grams each. 

Roll each of your six pieces of dough on the counter until you have 6 balls. 

Add each ball to a parchment-lined sheet pan. 

Press down the top of each ball with a lightly oiled hand or the lightly oiled bottom of a mixing cup or small pan. If you want very tall and round buns, don't flatten at all. If you want medium height but still rounded buns press down only a little. If you want flat buns like you see in fast food restaurants, you should flatten each one fairly flat. 

Cover your buns with lightly greased plastic wrap or a second sheet pan flipped over on top of the pan you're using. 

Let shaped buns rise for another hour or hour and a half until they are nearly doubled in size.

Near the end of your final proofing time, preheat your oven to 375 degrees F (177 C). 

Create an egg wash in a small bowl by whisking together 1 large egg and 1 tablespoon of water. Paint the egg wash on each piece of shaped dough with a brush or very carefully with a spoon. Once all of the buns are painted with egg wash, sprinkle sesame seeds on top of each one. The egg wash helps with browning and it will "glue" the seeds to the bun. 

Bake your buns for 15 to 20 minutes rotating the sheet pan halfway through the baking time. I typically bake for 7 minutes and then rotate and come back in another 7 minutes to see what my level of brownness looks like. At that point your buns should be done all the way through, you're just trying to get them to the color you prefer. 

Remove buns from the oven and allow to cool before slicing.