Firehouse BBQ Sauce

1 (46 fluid ounce) bottle ketchup
1 1/2 cups apple cider vinegar
2 cups packed brown sugar
1/2 cup butter, cut into pieces
2 tablespoons red pepper flakes, or to taste

Pour ketchup into a large saucepan. Pour vinegar into the ketchup bottle, shake to loosen any remaining ketchup, and pour into the saucepan. Save the bottle, and clean. Stir in the brown sugar, butter, and red pepper flakes. Cook over medium heat until almost boiling, but do not boil. For convenience, refrigerate leftover sauce in the clean ketchup bottle.




1