The 30-year-old actress chose one of Kutcher's favourite restaurants in Hollywood for his birthday dinner, reported Us magazine.
She invited his friends to the French restaurant La Poubelle which she rented to celebrate the occasion.
"It was very small. Ashton has been going there for year and years. It's one of his favourite spots, like home, so it was comfortable and they were well taken care of," source said.
The actor did not want a lavish party and was mesmerised by the surprise.
"They had been planning it for awhile. Ashton didn't want a huge thing but they got a good group together. He's certainly changed from how he used to like to party years ago. This was more of a wholesome situation. They had a great time," the source added.