I prefer letting my graham cracker toffee bars set at room temperature and not in the fridge while they set up after being in the oven. When you refrigerate these toffee bars, you'll see condensation on them after the chocolate has been refrigerated and then comes to room temperature. But this is merely a cosmetic issue. If you’re in a hurry, you can expedite the cooling process by refrigerating, noting that condensation will gather on the bars.