Create inner Shadow in a UIButton

Create inner Shadow in a UIButton

I want to create a inner Shadow. The Shadow should be inside the Button on
the bottom. But i can only create a Shadow outside of the Button, like
this:
self.button.layer.masksToBounds = NO;
self.button.layer.shadowColor = [UIColor blackColor].CGColor;
self.button.layer.shadowOpacity = 0.8;
self.button.layer.shadowRadius = 12;
self.button.layer.shadowOffset = CGSizeMake(50.0f, 50.0f);
Whats wrong?
Thanks!!