He continued: "My studio and I had the honour (and the best time) collaborating with her on her special gown. Best of all she is dedicated to using her privilege to help our world and its people with her project @the_anti_slavery_collective. Happy Birthday! @princesseugenie."
Following the big day, the American fashion designer shared never-before-seen photo of the custom design, revealing every stunning detail. The back was designed with buttons trailing down the spine with the silky fabric flowing out from behind in the shape of a cape.
Just like her first dress of the day, the gown was created with a low back to avoid covering Princess Eugenie's scar.
Eugenie also posted in honour of her birthday, writing: "My 28th year was the best one yet. Now to being 29...Thank you for all the birthday wishes."
Jack and Eugenie, who had been together eight years prior to their wedding, met during a ski holiday in Verbier, Switzerland, where Jack was working as a waiter at the time.
"It was love at first sight," Princess Eugenie has previously said. "Jack's the kind of guy, when you're at a party and you can't find anyone to talk to and you start to panic and you need help, he'll walk in and he just makes everyone feel so special," she continued. "He'll scoop you up and talk to you and make you feel a million dollars."
WATCH: The Special Meaning Behind Princess Eugenie's Wedding.