Whisk sweet chili sauce, sambal oelek, soy sauce, rice vinegar, and sugar until dissolved. Add garlic, ginger, and red pepper flakes. In a separate bowl, whisk cornstarch with water or broth until smooth, then stir into sauce mixture. Set aside.
Pat chicken dry with paper towels. In one bowl, whisk cornstarch with salt, pepper, garlic powder, and cayenne. In a second bowl, beat the eggs. Line a plate with paper towels for draining.
Dip chicken pieces in beaten egg, letting excess drip off. Immediately coat in cornstarch mixture, pressing gently to adhere. Shake off excess and set aside. Repeat with all chicken.
Heat 1/2 inch of vegetable oil in a wok or skillet over medium-high heat to 350-375°F (175-190°C). Test with a pinch of cornstarch — it should sizzle immediately.
Add coated chicken in a single layer without overcrowding. Fry in 2-3 batches for 5-6 minutes, turning occasionally, until deep golden brown and internal temperature reaches 165°F (74°C). Drain on paper towels.
Pour off most of the oil, leaving about 1 teaspoon. Return pan to medium heat, add white parts of green onions, and cook 30 seconds until fragrant.
Stir sauce mixture (cornstarch settles), pour into pan, and cook 1-2 minutes, stirring constantly, until glossy and thickened enough to coat a spoon.
Return fried chicken to pan and toss 30-60 seconds until fully coated. Transfer to serving platter or over steamed rice. Garnish with green parts of onions and sesame seeds. Serve immediately.