madonnatwitter.jpgcourtesy Twitter

Kanye West's g-friend Amber Rose was WAY star struck the other night at the Costume Institute Gala when Madonna herself GRABBED MS. ROSE'S BUTT!

She was so star struck, in fact, that she posted evidence of the grabbing on her Twitter page for the world to see.

Kanye, meanwhile, seemed to take it all in stride ... look at that look he gives the camera!