Heat broth in a small saucepan. Stir in green onion, garlic, soy sauce, chili powder, and brown sugar. Bring mixture just to boiling; reduce heat. Simmer, uncovered, for 2 to 3 minutes, stirring often. Stir in peanut butter, 2 tablespoons coconut milk, and ginger; heat through. Thin sauce with remaining coconut milk, if desired. Serve at room temperature.
To store, refrigerate in an airtight container for up to one week. Makes about 1/2 cup.