Gather all the marinade ingredients. Slice the lemongrass into thin circular pieces starting from the bottom of the stalk. Keep slicing until the inner purple circles disappear. Mince the shallots and peel the garlic, then put all 3 ingredients into a mortar and pestle and grind until the mixture is a rough paste. If you don't have a mortar and pestle, you can just mince the aromatics with a knife.
Mix the paste with the fish sauce and sugar and stir thoroughly until well-mixed.
Weigh out 8oz of chicken and cut the chicken into small, bite-sized pieces, trimming off any excess fat along the way. Mix the chicken into the marinade and let marinade for ideally 2-4 hours or overnight. In a pinch, you may be able to get away with 30 minutes, but the flavor may not be as pronounced.
Line your air fryer basket up with a pieces of aluminum foil and spray with cooking spray. Add the chicken pieces in one layer. Air fry at 380 degrees Fahrenheit for 10 minutes. Then open the basket, flip the pieces over, and air fry for another 8 minutes at the same temperature.
Remove the chicken pieces from the air fryer, pair with your sides, and enjoy your crispy lemongrass chicken!