Kourtney was joined by partner Scott Disick and their children Mason and Penelope in celebrating Kylie's 15th birthday in Malibu. Credit: Splash News and Pictures

Kourtney Kardashian delivered baby No. 2 on July 8, and on Friday, she had a different kind of delivery to make: birthday presents!

The reality star, who gave birth to daughter Penelope Scotland just five weeks ago, was all smiles while walking hand-in-hand with two-year-old son, Mason Disick, as they arrived at Kylie Jenner's 15th birthday party in L.A.

Dressed in black skinny jeans, black flats, and a loose-fitting royal blue polka-dot blouse, the E! star had that unmistakable new mom glow.

On Friday, the 33-year-old commemorated her little sister's milestone with a shout out on her personal blog. "You are filled with so much fun and happiness but so nurturing and understanding at the same time. You have wisdom beyond your years and I'm proud to call you my sister. I love you madly. I hope all of your birthday wishes come true," she wrote.

In the new issue of Us Weekly (out now) Kardashian introduces her and boyfriend Scott Disick's newest edition to fans.

Gushes the ecstatic second-time mom (who shares Us' cover with sleepy Penelope and a beaming Mason): "Nothing could prepare me for how hard I fell in love with her."