In a small bowl, stir together the peanut butter, soy sauce, sesame oil, vinegar, sugar, and garlic, and whisk in the water or broth, blending until smooth. Stir in the cilantro, if using, and chile oil to taste.

If desired, thin the sauce to the desired consistency with a tablespoon or two of additional water or broth.

Cover and refrigerate until ready to serve, up to 3 days.