Whisk the peanut butter, hoisin, lime juice, sugar (if using), garlic, and chili sauce in a bowl.
Add warm water 1 tablespoon at a time , whisking until smooth after each addition. The sauce will seize and look lumpy at first — keep whisking, it smooths out. Stop when it reaches a thick-but-pourable, heavy-cream-like consistency that coats the back of a spoon.
Taste and adjust: more lime for brightness, more hoisin for savory depth, more chile for heat. If it thickens as it sits (it will), whisk in warm water a teaspoon at a time to loosen.
Transfer to a serving bowl and top with crushed peanuts. Serve at room temperature.