Heat 2 tablespoons cooking oil in a large skillet over medium-high heat.
Add the chopped yellow onion and cook, stirring occasionally, until soft and translucent, about 5 minutes.
Stir in the minced garlic and minced ginger and cook for 1–2 minutes until fragrant.
Add the bite-sized chicken pieces and cook, stirring occasionally, until lightly browned on the outside, about 3–4 minutes.
Stir in the sweet chili sauce, tomato sauce (or ketchup), crunchy peanut butter, chili powder, and dark brown sugar (if using) until combined.
Pour in the unsweetened coconut milk and add 1 teaspoon salt, then bring the mixture to a boil.
Reduce heat to medium-low, cover, and simmer for 20 minutes, stirring occasionally, until the chicken is cooked through and flavors meld.
If the sauce is thinner than you prefer, whisk 1 tablespoon cornstarch with 2 tablespoons water, stir into the skillet, and simmer about 1 minute until thickened.
Taste and adjust salt if needed, then serve the chicken over steamed rice and garnish with chopped cilantro and roasted peanuts.
