Combine all ingredients, except soy sauce, and butter, in a medium saucepan.
Simmer on low for 20 minutes, making sure to stir occasionally.
Add the remaining ingredients and simmer for 10 more minutes.
Allow sauce to cool slightly before using. If making ahead of time, store in an airtight container in the refrigerator for up to 5 days after preparation.
Mustard
If using mustard powder instead of prepared mustard, mix 1/3 cup each of mustard powder, water and (cider) vinegar in a non-metalic bowl.
Stir well, then leave to stand for 30-60 minutes, stirring occasionally.
Optionally you can add a teaspoon of honey to sweeten.