Special equipment: 1 (2 1/2 or 2 3/4-inch) biscuit cutter; steamer, steamer basket or bamboo steamer
For the pork buns: Bring 2 cups water to a simmer in a pot that will fit a steamer basket or a bamboo steamer.
In a medium bowl, mix together the pork, ginger and scallions. Set aside
Working with 1 dough canister (10 biscuits) at a time, roll out the biscuits with a rolling pin until they are about 3 to 3 1/2 inches in diameter. Place 1 heaping tablespoon pork mixture in the center of 5 of the biscuits. Cover each with a remaining rolled-out biscuits. Gently press the biscuits together all the way around the filling and to the edges. Place a (2 1/2 or 2 3/4-inch) biscuit cutter on top so that it is centered on the bun. Press down on the cutter to form a clean, sealed edge and remove the excess dough from around the outside of the biscuit cutter. (If you do not have a biscuit cutter then just pinch together the edges of the buns with your fingers.) Place the sealed buns onto a sheet tray and cover with plastic wrap. Repeat with the remaining ingredients.
Place the buns into the steamer basket. Depending on the size of your steamer, you may have to cook the buns in batches of 5. Place the steamer basket over the pot of simmering water, cover and steam, about 10 minutes. The buns will be puffed and the tops will look somewhat dry and shiny. Let cool 1 to 2 minutes.
While the buns are steaming, make the hoisin dipping sauce: In a small bowl, stir together the hoisin, soy sauce, 2 tablespoons water and the vinegar. Serve with the buns.