Happy 23rd birthday, Anastasia Karanikolaou!

Kylie Jennertook to Instagram on Tuesday to celebrate her best friend's birthday, and alongside a series of photos of the duo, she described Stassie as the

Happy 23rd birthday, Anastasia Karanikolaou!

Kylie Jennertook to Instagram on Tuesday to celebrate her best friend's birthday, and alongside a series of photos of the duo, she described Stassie as the "rarest of all time" and "truly a gift to this world."

Even more sweet, Kylie called Stassie "the best aunty to my babygirl," Stormi Webster.

"i can't believe we are celebrating your 23rd birthday," Kylie continued. "i cherish every year we've spent together and i thank God for putting you in my life. i wouldn't be able to do it without you. i have your back till the end of time. today and every day.. we celebrate you."