In a medium saucepan, whisk together all ingredients: apple cider vinegar, water, brown sugar, smoked paprika, red pepper flakes, black pepper, salt, and optional cayenne pepper.
Bring the mixture to a gentle simmer over medium heat, stirring occasionally to dissolve sugar and salt.
Reduce heat to low and simmer for 5-10 minutes, allowing spices to bloom and flavors to integrate.
Remove from heat and let the sauce cool completely to room temperature for deeper flavors.
Once cooled, transfer the sauce to a cruet or squeeze bottle for easy serving.
Notes
For best flavor, let the sauce sit overnight in the refrigerator before serving.