Melt butter in a large sauce pan. Add onions and cook until tender.
Add the garlic and chili peppers and continue to cook for an additional 2-3 minutes. Add the chili powder and stir well. Cook over moderate heat until the chili powder's flavor is developed.
Add the remaining ingredients and simmer for approximately 45 minutes.
Mix approximately 2 tablespoons of cornstarch with 1/4 cup of water until it is smooth. Stir the slurry into the BBQ sauce to adjust the thickness of the sauce. Be certain to bring the sauce back to the boil before cooling to cook out the starch of the cornstarch.
Sauce can be used at this point or be stored under refrigeration for up to one week.