The 25-year-old Algerian model, who dated the mother-of-three from 2016 to 2018, shared a snap from the event, which was held in Los Angeles on Thursday, April 18.
Posting on his Instagram Story, Younes gave fans a glimpse of the reality TV star's personalised napkins, which read, "Happy birthday Kourt."
He then shared a video from the dance floor with the caption: "Ya know what time it is (sic)."
In another picture, posted by businessman Philip Riportella, Younes poses with his arm around the birthday girl, who wore a sheer black Versace dress for the occasion.
Th "Keeping Up with the Kardashians" star Kourtney broke up with Younes in August (18), but fans are convinced that the couple may be back together again after the model commented on one of her Instagram posts on 12 April.
"Ya look good Minnie Mouse (sic)," he wrote on a photo of Kourtney posing in a red leather dress.
Kourtney quickly replied with a flirty, "Merci (thank you), Mickey," apparently using the nicknames they had shared for one another during their relationship.
Since their split, Kourtney has been linked to 21-year-old actor and model Luka Sabbat, who was also in attendance at her 40th birthday party.
She and Younes have yet to comment on rumours of a reconciliation. Their relationship marked Kourtney's first serious romance since parting ways with her boyfriend of nine years, Scott Disick, the father of her three children, in 2015.