What does adding butter to toffee do?
Butter largely acts to prevent the crystallization of sugar in toffee. This leaves the toffee smooth and breakable.


  Fats in candy serve a similar purpose. Fatty ingredients such as
  butter help interfere with crystallization—again, by getting in the
  way of the sucrose molecules that are trying to lock together into
  crystals. Toffee owes its smooth texture and easy breakability to an
  absence of sugar crystals, thanks to a large amount of butter in the
  mix.
  There's more about the role of ingredients in candies here.


You should still be able to make stickjaw (or, if not authentic stickjaw, at least a more sticky) toffee. The consistency of toffee has more to do with the temperature you bring the toffee to while cooking. Here's a chart detailing the various stages of candy temperatures. For a sticky toffee you'd most likely want to bring your toffee to the soft-crack stage.

Here is a recipe for authentic, old-fashioned, stickjaw toffee but without butter.