I usually use rolled oats or quick cooking (but not instant) oats for oatmeal cookies. Can they be made with steel cut oats?
Steel cut oats take a lot longer to cook than rolled or quick oats. It's unlikely they will completely cook before the cookie is done. You could try it, but I'd recommend presoaking them in hot water for an hour, or maybe even parboiling them for a few minutes. You'd need to adjust your wet ingredients accordingly as well.
I have been considering trying toasted steel cut oats as an ingredient in or as a garnish on cookies. I haven't tried it yet, but I plan on spreading a thin layer of the oats on a sheet pan and toasting lightly in a low oven then allowing them to cool. I'd probably try them with my usual oatmeal raisin cookie recipe, or one that usually calls for some cereal flakes as an ingredient. If I try the garnish idea I might use a shortbread cookie recipe and roll the dough balls in the toasted oats or just dip the tops.