Crab Rangoon is a Chinese-American favorite, commonly found on most take-out menus. You can use real crabmeat (I like Jonah crabmeat for its sweetness) or canned or imitation crabmeat, in which case I prefer the Japanese kani crab stick you would use for sushi. — Lee Anne Wong, Dumplings All Day
Combine the crabmeat, scallions, cream cheese, mayonnaise, ginger, salt, sugar, and pepper in a mixing bowl and stir until well blended. Refrigerate the filling for at least an hour.
Place 1 tablespoon of the filling in the middle of the square wrapper and wet the edges of the wrapper using a pastry brush or your finger. Bring the two opposite corners together over the filling, using water to pinch the two corners together. Bring the remaining two corners to the center and seal all the edges together with a gentle pinch. You may use this technique with round wrappers to achieve either a square (four seams) or a triangle (three seams).
Preheat a few inches of oil to 340 degrees F in a heavy-bottomed pot. Add the dumplings, a few pieces at a time, to the oil. Gently turn the dumplings in the oil, cooking until all sides are golden brown, about 3 minutes. Drain the dumplings on paper towels. Make sure the heat of the oil stays at 340 degrees F while frying the dumplings, and return the oil to this temperature before adding new dumplings.
To make the sauce, combine the vinegar, sugar, ketchup, soy sauce, and cornstarch mixture in a small saucepan. Bring to a boil while whisking and then reduce to a simmer. The cornstarch will thicken the sauce rapidly; whisk until smooth and then remove from the heat.
Serve the dumplings warm with the sweet and sour sauce.