Warm the oil in a covered saucepan on low heat.
Add the onions, bay leaf, and salt and cook for 5 minutes.
Stir in the hot peppers or cayenne, garlic, ginger, coriander, if using, and tamarind concentrate.
Continue to cook for 5 minutes, stirring occasionally.
Add the water, coconut milk, and vinegar, increase the heat to a low simmer, and cook, covered for 3 minutes.
Stir in the peanut butter and sugar until the sauce is smooth.
Bring to a simmer and cook, covered, on low heat, stirring occasionally, for 10 minutes.
Remove the bay leaf.
Stir in the soy sauce, and add more to taste.