Combine the sweet chili, lemongrass, fish sauce, garlic, ginger and garlic chives in a pot and warm over medium-low heat. Mix together 2 cups of the flour, the salt, dark chili powder, garlic powder and paprika in a large bowl. Beat the eggs with the milk in another bowl, incorporating the other 2 cups of flour a little at a time, constantly whisking until the batter is twice as thick as the milk.
Dredge the chicken thighs in the seasoned flour mixture, then dip in the flour-egg mixture, and toss into the rice flour to finish. Fry the chicken, in batches if needed, until the internal temperature reaches 165 degrees F. Toss in the warm lemongrass sauce and serve.