"My baby girl turns 5 today. Never did I think back then that I would have my best friend for life!" Kardashian captioned a photo from North's joint birthday party with cousin Penelope Disick.
"I'm so honored to be your mom Northie! You have brought more joy into my world than I ever could have imagined. I can't believe your so big now!," she added.
"The luckiest girl in the world to be best friends with your cousins and have the most magical parties together! I love you so much birthday girl!"
Kardashian helped throw a unicorn-themed party for North and Penelope, the daughter of her sister Kourtney Kardashian, earlier this month.
Kardashian shared videos Thursday on Instagram Stories of herself treating North to ice cream ahead of her birthday.
"We are making our own ice cream today since we're in New York for North's birthday," she said during a visit to CoolMess. "We're making chocolate and vanilla."
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Kardashian's mom, Kris Jenner, also wished North a happy birthday Friday on Instagram.
"Happy birthday to our sweet angel North! I can't believe you are 5 today!" Jenner captioned a slideshow of photos.
"I love you so much, you are the greatest blessing, and it brings me so much joy to watch you grow into such a beautiful, fun-loving, spirited, kind, smart and funny little lady! Love, Lovey #HappyBirthdayNorth," she said.
Kardashian is parent to North, 2-year-old son Saint West and 5-month-old daughter Chicago West with West.
Her sister Khloe Kardashian gave birth to her first child, daughter True Thompson, in April, and shared new photos of the infant this week.
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