Prepare a grill for high heat (450° to 550°). In a small saucepan over medium heat, cook garlic and peanut butter together until simmering, about 4 minutes. Whisk in sugar, chile flakes, soy sauce, and 2/3 cup water. Reduce heat and simmer, stirring occasionally, until starting to thicken, about 10 minutes.
Meanwhile, rub chicken with oil and grill until golden brown and almost cooked through, turning once, about 6 minutes total. Brush about 1 tbsp. peanut sauce over each chicken piece and cook just until sauce starts to caramelize, about 2 minutes. Serve with steamed vegetables and remaining sauce on the side.
Note: Nutritional analysis is per serving.