Dice the blackened chicken breasts into small, bite-sized pieces.
Chop the cooked bacon into small bits.
Dice the tomatoes into small cubes.
Finely chop the Romaine lettuce.
Slice half an avocado thinly and set aside for topping.
In a large mixing bowl, combine the chopped chicken, bacon, tomatoes, and lettuce.
Add the Kewpie mayo, salt, and pepper.
Toss everything together until evenly coated and well mixed.
Toast the brioche slices until golden brown.
Pile the chopped chicken BLT mixture generously onto one slice of toasted brioche.
Lay the avocado slices on top of the mixture.
Top with the second slice of brioche.
Slice the sandwich in half and enjoy immediately.