In a small bowl, combine the hoisin sauce, chili oil, and toasted sesame oil. Stir well until the mixture looks smooth and evenly blended.
For a nuttier and fuller-bodied sauce, add the smooth peanut butter at this stage. Mix thoroughly until the peanut butter is completely incorporated and no streaks remain.
Gradually pour in the water, beginning with small additions, and stir after each one. Continue until the sauce reaches your preferred consistency, using up to 6 tablespoons in total. If peanut butter has been included, additional water will usually be needed to loosen the mixture.
Transfer the sauce to a serving bowl once the texture is to your liking. Garnish with crushed peanuts, toasted sesame seeds, chopped cilantro, or green onions, if desired.
Serve immediately as a dip or drizzle over pork belly and other dishes.