This recipe makes 2 rolls if you use two 9 1/2 x 13 x 1 inches pans or known as quarter sheet pan here in the U.S. You can also use one half-sheet pan, 18 x 13 x 1 inches. I use one half-sheet pan. The dough doesn't fill up the pan entirely, about 90%
Put all ingredient in a mixing bowl of a stand mixer fitted with a dough hook attachment
Mix on lowest speed for 2 minutes and then increase speed to 2 (on KA) and knead for about 5 minutes. Add softened butter and increase speed to 4 and knead for about 5-7 minutes or until the dough reaches the windowpane stage. You need to stop several times to scrape the side of the bowl and start kneading again. When you pinch off a small amount of dough and slowly and thinly stretch it, it won't break. If it breaks, knead a bit longer. The dough should be smooth and very elastic and sticky to the touch. Don't add any flour
Lightly oil your hands and pick up the dough and round it up into a smooth dough bowl
1st proofing:
Lightly oil the bowl and put the dough in there, cover with a plastic wrap and let it proof in a warm place until it doubles in size
Shaping:
If using two pans, divide the dough into two equal-weight dough. Deflate the dough by punching down the dough, releasing all the air. Pull the edge to the center to form a dough ball. Cover and let it rest for 15 minutes to relax the gluten so we can easily roll the dough out
Line the pans (if using two pans) with parchment paper. Roll the dough out on the parchment paper to about the same size as the pan, slightly bigger is fine as they will shrink a bit as you stretch it. Make sure you roll it out with the same thickness throughout. Transfer to a baking pan and stretch the four corners to fill up the pan. It won't fill up the pan entirely and that's okay, just do your best. If the dough keeps retracting, give it a 5-10 minutes rest and then stretch again
2nd proofing:
Cover and let the dough proof again in a warm place, until it is puffy. It may take about 50 minutes to 1 hour or a bit longer, depending on the temperature. Preheat the oven to 375 F (190 C) for a conventional oven and 350 F (180 C) for a convection oven, 15 minutes before the end of 2nd proofing
Prepare topping:
While waiting for the dough to proof, cut the green onion into fine pieces. Sprinkle with salt.
Check on proofing:
When you gently poke on the dough, it should bounce back very slowly and leave some indentation. The dough is done proofing. If it bounces back immediately, proof it a bit longer
Use the tines of a fork and gently poke the dough all over the surface to prevent the dough from bubbling up too much
Whisk egg with water to make the egg wash and gently brush on the surface of the dough
Sprinkle with white sesame seeds and green onion. Use your finger to gently press on them a bit so they stick to the dough
Bake the bread:
I recommend baking one tray at a time. Place the baking pan on the middle rack and bake for 10 minutes. The bread will still look kinda pale at this point. Turn the upper heat on, if you have one and bake for another 2-3 minutes or until nicely golden brown.
If your oven doesn't have the control for upper or lower heat, turn the broiler on high and broil for 2-3 minutes or until golden brown. Do not walk away
Remove from the oven and immediately cover the bread with a large clean kitchen towel. Place a cutting board or something flat on top and flip over. Remove the parchment paper and immediately roll the bread up using the towel, like a Swiss roll. Let it stay rolled up in the towel for 10 minutes.This will keep the bread moist and soft, and prevent the bread from cracking when you roll it
Assembling:
Gently unroll the bread and apply a thin layer of kewpie mayo to the surface. Mound the meat floss in the middle. Roll the bread up again. You shouldn't have much problem since it has been pre-rolled before
Serving:
Trim off the two ends a bit to make it look neater. Cut into 6 serving slices per roll. or 12 rolls if making one large roll. Spread a thin layer of kewpie mayo on the cut side of each slice and then dip into meat floss to cover both sides
How to store:
I suggest only apply the meat floss and kewpie mayo on the cut side when you are ready to serve. Let the buns cool down completely and then keep in an air-tight container for a day or two.
For longer storage, put them in a freezer bag, push all the air out and seal. Keep frozen for one month. Simply thaw at room temperature for 15-20 minutes and then wrap in a foil and reheat in the oven at 350 F for 10 minutes or until just warm and then apply kew pie mayo and meat floss on the cut side and serve