In a bowl, mix together the flour, cornstarch, and five spice powder. Season with salt and pepper, to taste. Fill a large, cast-iron skillet about ½ full with cooking oil.
Place over medium heat until very hot; a pinch of flour dropped into the skillet should sizzle. Roll the chicken pieces into the breading and shake off excess flour.
Drop the pieces into the hot oil; do not crowd. Deep-fry until golden brown, about 20 minutes. Drain the fried chicken and let rest until cool enough to handle. Cut into bite-sized pieces.*
In a bowl, mix the wasabi paste with the mayonnaise. (Add more wasabi paste if you want a bolder kick.) Arrange the chicken slices on the cucumber and add a dollop of wasabi paste onto each piece.