Nothing is more annoying than lovingly making a batch of homemade goodies like brownies only to find them rock hard the next day. A number of reasons can cause brownies to harden, from how you prepared the brownies to how you store them. While you may not be able get them as soft as they were when you took them out of the oven, you can restore some of the lost moisture so they're edible and enjoyable once again.
Place individual hardened brownie squares in an airtight plastic container and put one or two slices of bread on top of them. Cover the container and leave it overnight. The brownies will absorb the moisture from the bread and should be significantly softer the next day. This method is more commonly used to soften cookies, but it also works for brownies and other baked bars.
Microwave the brownies right before you plan to eat them. Wrap the squares individually in damp paper towels and place them in the microwave. Heat for 5-second intervals until the brownies are sufficiently softened. Eat immediately, or they will harden again as they cool. Do not leave the brownies unattended in the microwave, as they can easily burn or become toughened and inedible.
Prevent your brownies from ever getting hard in the first place. Don't over-bake them by checking their doneness several minutes before the recommended baking time. Wait to cut the brownies until they have cooled sufficiently. Cut and wrap each square individually in plastic wrap to help keep in the moisture and store them at room temperature for up to four days. You can also freeze the brownies for four to six months, wrapping them not only in plastic wrap but in a second layer of aluminum foil. To reheat the frozen brownies, unwrap and let them thaw at room temperature.