It's both! 'Chilli' is the spelling commonly used in British English, whereas 'chili' is the common spelling in American English. I have readers in countries that use both of these forms, so I've used both versions on my site. Additionally, 'chili' is also name for a type of thick stew in the U.S., and I wanted to differentiate the type of chili being used in these eggs (i.e. it is not stew!).