Preheat oven to 350 degrees. Line a large baking sheet with a silicone baking mat or parchment paper. Sift all the dry ingredients into a large mixer bowl until there are no lumps. Turn mixer onto low. Gradually add the apple cider vinegar and the room temperature water (if using the egg white powder)
If using fresh egg whites instead of the powder add the egg whites now. Increase speed to medium-low and allow the wet ingredients to combine until all the dry ingredients are moist. Add the HOT milk and beat on medium-low until a wet sticky dough (batter) has formed.
Scrape down the sides of the bowl and push the dough to the center of the bowl. Allow the dough to sit for 15 minutes to absorb some of the liquid. After 15 minutes the dough will still be wet but should have absorbed some of the liquid. Use a large ice cream scoop or 1/4 cup measuring cup and scoop the dough out onto the prepared baking sheet.
Leave space between each scoop to expand. Use a butter knife and gently smooth the scoop into a circle shape. Bake in preheated oven at 350 degrees for 25 to 30 minutes or until golden. Remove from the oven and allow to cool on the pan for 10 minutes. Transfer to a wire rack and allow to cool completely before slicing.
When ready to use eat as is or turn the milk buns on their side and cut to create a top and bottom for each bun. For warm milk buns warm in microwave 10 to 20 seconds or warm in the oven a 3 to 5 minutes until warm.