Homemade White Castle Dusseldorf Mustard BBQ Sauce

Ingredients

• 1/2 cup Dusseldorf mustard (or substitute with spicy brown mustard)
• 1/4 cup yellow mustard
• 1/2 cup apple cider vinegar
• 1/3 cup light brown sugar, packed
• 2 tablespoons honey
• 2 tablespoons ketchup
• 2 tablespoons Worcestershire sauce
• 2 teaspoons smoked paprika
• 1 teaspoon garlic powder
• 1 teaspoon onion powder
• 1/2 teaspoon freshly ground black pepper
• 1/2 teaspoon kosher salt
• 1/4 teaspoon cayenne pepper (optional, for heat)
• 2 tablespoons unsalted butter