Do you have to ignite brandy to burn off alcohol?

Yes, igniting brandy can help burn off some of the alcohol, but it's not a foolproof method. When heated, the alcohol evaporates, and igniting it can burn off a portion of the alcohol content, but not all of it. The duration of the flame and the temperature will affect how much alcohol remains. If you're looking to reduce the alcohol content significantly, cooking it for a longer period is more effective than simply igniting it.