Mix flour, cornstarch, baking powder, salt, water, egg, and sesame seeds. Let sit for 15 minutes.
Wash the chicken breast fillets and pat dry with kitchen paper. Dice into nugget-sized pieces and season with salt and pepper.
In batches, coat the chicken pieces in the batter and remove, letting excess drip off.
Heat 1 ½ liters oil to 160–180°C (320–350°F) over medium-high heat. Fry the chicken nuggets in batches for 5–7 minutes until golden brown and the internal temperature reaches 74°C (165°F).
Remove with a slotted spoon and drain on kitchen paper.