Combine the two flours in a mixing bowl. Add hot water and mix thoroughly; when it is combined well, and crumbly in texture, add cold water and continue mixing with a spoon or dough hook if you’re using a mixer. When dough cool enough to handle, begin kneading; the dough should be soft and a slightly sticky. If doing this by hand, you should knead the dough for fifteen minutes or so, adding flour if it begins to stick to your hands. Be aware, however, that a soft, very stretchable dough is required to make xiao long bao, so too much flour will make a stiff, difficult material to work with. On the other hand, the challenge is to make a dough that is strong enough to contain a hot soup during the time it is being cooked and served. To balance both concerns, I suggest kneading as much flour into the dough as it will absorb, then letting it rest in the refrigerator overnight, wrapped in plastic. Bring to room temperature before using.
At any time after you’ve mixed the dough and filling, you can make and steam a small test dumpling to check seasonings, stock to filling ratio, and a sense of whether the dough is right for the dumpling.