can I substitute parchment paper for the foil in a bar cookie recipe
Short answer for a short question: Probably. Assuming you're using it to line the pan, paper might leave the cookies less browned than you'd like, but it will provide a physical barrier between cookies and pan just fine. If you're using it to cover the bar cookies to keep them warm, foil works a lot better than paper. If you're cooking the cookies in a makeshift pan made solely out of foil by shoving the whole thing into a campfire, paper will definitely not work.