Demi Moore's daughter Rumer Willis sparked controversy by celebrating her birthday with a cake shaped like a gun.

The actress, whose father is Hollywood action man Bruce Willis, turned 26 on Saturday (16Aug14) and her famous mother marked the big day by ordering a cake shaped like a pistol decorated with the words "Happy birthday Rumer... Bang Bang".

Willis posted a picture of the cake on her Instagram.com page along with the caption, "Best birthday cake ever. Thank you mama for making this day do (so) special."

However, the picture sparked a slew of negative comments from the star's Instagram followers, who branded the stunt "sick".

The cake is believed to be a reference to Willis' turn in Quentin Tarantino's stage musical tribute For the Record in which she sings the track Bang, Bang (My Baby Shot Me Down).