For the dumplings: In a large bowl, combine the chicken, scallions, garlic, ginger, and soy sauce, mix well. Season with salt and pepper.
Lay down the dumpling wrappers on a dry, clean surface, brush lightly with water around the edges, and place a teaspoon of filling in the center. Fold one point of the wrapper over the filling to the opposite point of the wrapper, and press tightly along the edges to seal them shut. Set aside on a piece of parchment paper.
Place a saucepan of water on medium heat with a steamer basket or perforated strainer over top cover with a lid - If you don't have that kind of setup you can heat a small amount of water in a large sauté pan to a low simmer.
Grease the perforated strainer with non-stick spray and place the dumplings in a single layer, or in batches if needed (being careful not to overcrowd the pan) and cover with a lid. Steam for approximately 6-8 minutes, to an internal temperature of 160°F using a thermometer, turning over about halfway through. Remove with a slotted spoon onto a paper towel to drain any excess water. Season with salt.
For the sauce: In a saucepan over low-medium heat, whisk together all ingredients until the sauce is thick and smooth.
Spoon the sauce over the dumplings and serve. Garnish with scallions.