The best way to prevent brownies from getting hard overnight is to bake and store them properly. When brownies are ready to be pulled from the oven, they will be set around the edges and when a toothpick is inserted in the center, it should come out with a few chocolatey crumbs.  Then, once the brownies are cooled and decorated, it is important to keep them in an airtight container to trap the moisture and keep them soft and chewy. If they are left loosely covered, they will dry out quickly and become hard and stale.