Yes, they can. What I prefer to do is bake the brownies, cool them and cut them into circles. Then store the brownies in an airtight container until you are ready to ice them. The frosting can also be made earlier in the day, placed into the piping bag then stored in the refrigerator until you are ready to ice the brownies. Just remember to take the piping bag out of the refrigerator at least 30 minutes before frosting the cakes and allow it to come to room temperature.