This traditional Vietnamese dipping sauce is spiked with a swirl of piquant chile sauce. If you can't find sugarcane syrup, substitute one teaspoon brown sugar mixed with one-half cup warm water.
Put peanut butter, sugarcane syrup, soy sauce, and chile paste into a small pot. Cook, whisking, over medium-low heat until smooth, 1 to 2 minutes; if sauce is too thick, whisk in more soy sauce, 1 teaspoon at a time. Transfer to a small bowl; refrigerate until cold. Serve swirled with more chile paste. Sauce can be refrigerated in an airtight container up to 1 week.