In a medium-sized mixing bowl combine the diced chicken with sesame oil, garlic, 5 spice, salt and pepper. Leave to marinate for 30 minutes.
Add the cornflour to a large shallow dish. Take 2 1/2 tbsp of the cornflour and mix it into the chicken (I find this easiest with my hands). Once evenly combined, leave for a few minutes to allow the cornflour to soak into the chicken. It will be quite sticky at this point, which is fine.
Mix the egg into the chicken until evenly combined. One by one, take a piece of chicken, thoroughly coat it in the cornflour, then lightly shake and place on a baking tray. Although tedious, this step is crucial to ensuring the chicken comes out crispy. Really squeeze the cornflour into every crevice of the chicken to create lots of tiny flaky bits. If you run out of cornflour, just add more as needed.
Add around 3/4" oil in a heavy-based/cast-iron pan, or enough to cover a little over half the size of the chicken. Turn the heat to medium-high and allow the oil to reach 190C/375F. From there, and working in 2-3 batches, add the chicken and cook until golden and crisp on the outside and piping hot through the centre (approx 3 minutes on each side). Remove the chicken and place on a wire rack with kitchen roll underneath to catch excess oil. Bring the oil back to temp between batches.
Take 2 tbsp of the leftover oil and add it to a wok or deep pan over high heat. Once the oil is hot, add the onion, peppers, garlic and chillies and stir fry for 2-3 minutes until it all starts to soften slightly. Add in the chicken, spring onion, 5 spice, sea salt and white pepper and toss until everything is well-distributed.
Serve up immediately and enjoy!