Kutcher officially turned 35 on 7 February (13), and, to celebrate the milestone, his former That '70s Show co-star invited 50 of the hunk's best friends to Go Kart World in Carson last Saturday (09Feb13) for a special party.
In addition, Kunis had his favourite fast-food restaurant, Umami Burger, provide snacks for the attendees.
And it appears the group had a blast - a source tells Us Weekly magazine, "She went through so much effort to keep it a secret. He was genuinely shocked. After everyone yelled, 'Surprise!' he turned to Mila and kissed her. He was so touched. He kept looking at her and grinning (from) ear-to-ear. They had a great time."
The couple began dating last year (12) following Kutcher's split from estranged wife Demi Moore.