Make the gochujang mayo: Mix the mayonnaise, gochujang and lime zest and juice in a bowl. Set aside.
Prep the chicken: Season the breast halves with salt, pepper and a pinch of chilli flakes.
Set up the coating: Season the flour with salt and a pinch of chilli flakes in one bowl. Beat the eggs in a second bowl. Do not season the eggs. Crush the cornflakes to the size of panko crumbs in a third bowl, season with salt, pepper and a few chilli flakes.
Coat the chicken: Dip each piece into the flour, shake off the excess. Into the egg, shake off the excess. Press into the cornflakes, pushing them on firmly. One dredge only.
Fry: Heat a 2cm depth of oil in a large frying pan over medium-high heat. Lay the chicken away from you and fry for 2 minutes on each side until golden and crispy. Drain on kitchen paper.
Make the cucumber salad: Peel the cucumber into ribbons with a speed peeler, discarding the seeds. Toss with the spring onions, coriander, fish sauce, lime zest and juice.
Assemble: Split the buns. Spread gochujang mayo on the bottom halves. If using the peanut crunch, spread a little mayo on the outside of the top halves and dip into the crushed peanuts and crispy fried onions. Lay lettuce on the bottom, then cucumber salad, then the crispy chicken. Top with a little more sauce and close the bun.
