Robert Downey Jr and Susan

The 48-year-old actor was keen to have film producer, Susan – who turned 40 on November 6 – celebrate her milestone in style at luxury resort Cavallo Point near Sausalito, CA, among a number of A-Listers.

Hollywood stars who joined the happy couple included Justin Theroux, Gwen Stefani, Gavin Rossdale, Gwyneth Paltrow, Jon Favreau and Reese Witherspoon.

A source told E! News: "It was a great weekend for Susan, and Robert went all out to make sure she knows how special she is.

"Around 50 of their closest friends came up for the weekend and stayed at the hotel in historic houses. It was a very relaxing weekend and there was a lot of time for people to catch up."

On Saturday night they partied in the ballroom of the hotel and that was followed by a fireworks display. The celebrations were family-friendly with entertainment provided for children too.

The source added: "There were kids running around everywhere, and the whole weekend catered to families. They had lot of kids' entertainment and the children were included in all of the events, even the nighttime birthday party. The whole group was very tight and very friendly. There was lots of hugging and reminiscing with old friends."

On Sunday morning the group enjoyed brunch before singing happy birthday to Susan before leaving.
The sourced added: "When it was time to go on Sunday, everyone talked about how much fun they had and that it was too short. They wished it could have lasted longer."