Put 6 tablespoons of the light soy sauce, 6 tablespoons of the mirin and the brown sugar into a small saucepan. Heat over medium heat for 5 minutes, or until reduced by a third.
Transfer to a small jug and set aside. Clean out the pan.
Melt the butter in the small saucepan over medium heat, then add a third of the garlic and stir for 30 seconds. Add another third and stir for a further 30 seconds, then repeat with the remaining garlic.
Put the chile powder, a pinch of salt, the lemon zest and three quarters of the chives into a large bowl and pour over the garlic butter. Stir to combine.
In a separate bowl, combine the chicken with the ginger, the remaining 2 tablespoons of soy sauce and mirin, a big pinch of salt and pepper, and mix thoroughly.
Add the cornstarch and toss until the chicken is coated.
Heat the oil in a frying pan over medium-high heat for about 3 minutes. It should be 1/2-3/4 inches (1-2 centimeters) deep, so add more if you need to.
Carefully place the chicken in the pan and gently push the pieces apart to prevent them from sticking.
Cook for 5 minutes, then flip the pieces and cook for a further 5 minutes. Remove from the pan and toss in the garlic butter.
Put the rice into bowls and top with the fried chicken.
Push the chicken towards the edge to make a small well in the center and add an egg yolk.
Sprinkle with the remaining chives. Pour the sauce over the chicken and enjoy.