Step 1

Butterfly each chicken breast by cutting into one long side of breast and gradually slicing until you are almost all the way to the other edge (do not cut all the way through). Open breast like a book, as if the connected edge were the spine. If one side is thicker than the other, cover chicken with plastic wrap and pound until even.

Step 2

Whisk soy sauce, oil, five-spice powder, garlic powder, and cayenne in a large bowl. Add chicken and toss to coat. Cover and chill at least 1 hour and up to 12 hours.

Step 3

Whisk mayonnaise, fermented bean curd (if using), and soy sauce in a small bowl until sauce is smooth.

Step 4

Do Ahead: Sauce can be made 3 days ahead. Store airtight at room temperature. Cover and chill.

Step 5

Mix five-spice powder and cayenne in a small bowl.

Step 6

Fit a large pot with thermometer and pour in oil to come halfway up sides. Heat over medium-high until thermometer registers 350°.

Step 7

While the oil is heating up, whisk cornstarch, rice flour, and 1¾ cups water in a large bowl (the mixture should resemble thick tempura batter).

Step 8

Working in 4 batches, fry chicken, turning occasionally and adjusting heat to maintain oil temperature, until deep golden brown, about 5 minutes per batch. Transfer to a wire rack set over paper towels to drain. Sprinkle lightly with salt and spice mixture while still hot. Let cool 5 minutes.

Step 9

Spread sauce on top of chicken and sprinkle with scallions and cilantro. Sandwich between buns.