Blend all ingredients for mustard soy dipping sauce and set aside.
In a food processor, add crab, egg, ginger and sesame oil.
Pulse only a little bit to bring ingredients together.
Transfer to a large bowl and fold in scallions.
Season with salt and pepper.
In a food processor, add 2 cups crab mixture, egg and butter.
Pulse until the butter is incorporated but still visible (small pieces).
Place 1 tbsp of mixture on a wrapper and bring up the sides going around the mound.
Continue with the remaining mixture and wrappers.
Drop, open side up, from 8 inches on a flat surface to level out the shumai bottoms.
Steam shumai for 6-7 minutes and serve with dipping sauce.