Add a text to the button, so css would be applied around it. For example:
%button{:class => 'btn btn-default btn-lg dropdown-toggle', :type => 'button', 'data-toggle'.to_sym => 'dropdown'} MyButton
In your case, there is no text for the button. It requires at least one non blank character to apply styling .
Above will create a large size bootstrap button with the text MyButton