I just baked square cookies that did not hold their shape and puffed up.
Did you cool them before baking them?
What is your butter to flour ratio? (i.e. what recipe did you use)
Use a shortbread recipe with no eggs, no leavener, very little sugar, and enough flour to hold the dough together. Make sure your dough is chilled before rolling, Do not use warm cookie sheets. Good luck!
New to shortbread, but used this recipe during the holidays with really good (shapewise and tastewise) results. It worked well with my Brown Bag clay molds, silicone molds, clay cookie stamps, and cookie cutters. http://www.kingarthurflour.com/recipes/gingerbread-springerle-shortbread-recipe
Some cookies were backed with chocolate, and the combination of spice and chocolate was really tasty..
Once you have your cut out cookies on your pan, before baking, refrigerate them for at least 15 minutes to firm up the butter, this will help slow the spread while baking.
Also - I've had alot of luck with King Arthur Flour holiday butter cookie recipe - does not spread.