For the pork, I used a boneless butt/shoulder, cut into 1 inch by 1 inch and however long they are logs. Marinate for 2-3 days in your char siu marinade. Using a grill, bbq the pork, we like small really dark areas, yes they look a little burnt here and there, but are actually a combination of the sugar over-carmelizing and the char of the grill. In my circle of friends this is now known as "candy meat"!
After grilling, let them rest and then cube up the Char Siu Bao portion and set aside, (save any leftover for another meal!

For the filling sauce, in a small cup, mix the corn starch and water together to form a 'slurry' which will be used to thicken the filling sauce and set aside.
Add the rest of the ingredients to a small sauce pan, mix together and turn on the heat to medium until the misture is boiling, then turn the heat down to low, and add a tblspoon of the hot mixture to the slurry mixing while you pur it in, once mixed add back to the hot sauce pan miixture stirring while your adding it. now let the sauce come to a simmer and let it reduce until its super thick and sort of oozes, this will take between 2 and 5 minutes depending on the stove heat. Once thick set aside let cool, if you want, transfer the mixture to a different bowl to facilitate cooling.

For the Bao, Let the yeast bloom (5-10 minutes) after adding warm water and a tblsp of the sugar from the 70 grams.
Add all the flour to a mixer with a dough hook. (Or you can hand mix it) Add the salt and sugar mix until blended.
Now add half of the bloomed yeast/water mixture, mixing on low speed, then slowly add the remaining yeast/water until it is all incorporated and a dough forms, you may need a rubber spatula to scrape down the sides or stir up the bottom.
Now add push the softened butter into the dough and let the mixer run at slightly faster then medium speed for 4 minutes. Make sure the butter isn't pooling outside the dough and gets mixed in to it.
Remove the dough and hand kneed for a minute and form a ball, pulling the top to the bottom to create surface tension before resting in a lightly oiled bowl covered with a damp tea towel for 1½ hours. (There are plenty of YouTube videos with bakers forming the dough balls.)
After 1½ hours rising, divide the dough in to 6 parts for large baos (about 4 oz each) or for smaller baos divide into 8 pieces.

Putting it all together: Roll the bao pieces into circles 3-4 inches in diameter, then add a heaping tablespoonful of the filling into the center, pull up 2 opposite sides together without the filling touching the edges or it will not stick together, pinch the sides then follow the 2 remaining areas that are left and pinch shut, bring the pinched ends together crimping the dough together so they do not leak during baking and then the seem ends up at the bottom of the bun.
Put them on a cookie sheet lined with parchment paper and best if you have one insert a rack in the tray and add the parchment paper to the rack grid rack lined with parchment paper insted of the cookie sheet. This will prevent the bottom from burning. Let them rise for 30 minutes once filled.
After the 30 minutes, brush the tops and sides with the egg wash.
Put them in the center of a pre-warmed oven at 350 degrees F. for 15-20 minutes. I always take a look at them after 10 minutes in any case. You never know. They should look golden brown and delicious when they are done!