The underlying title label, if one exists, can be fetched using
Swift
var label: UILabel? = button.titleLabel
Objective C
UILabel *label = button.titleLabel;
This can be used to set the font of the title label, for example
Swift
button.titleLabel?.font = UIFont.boldSystemFontOfSize(12)
Objective C
button.titleLabel.font = [UIFont boldSystemFontOfSize:12];