Those hardened brownies you left out overnight without proper storage probably look and feel completely inedible. But these chocolate bricks can usually be revived and restored to their moist and flavorful glory with a simple fix -- sliced bread and an airtight container. Commonly used to soften brown sugar and cookies, this technique works just as well for brownies and other cookie bars and should have your brownies softened within 24 hours or so.
Place the hardened brownies in an airtight container. If you have to layer the brownies, add a piece of parchment paper between each layer.
Add one or two slices of bread on top of the brownies in the container. If you are using a wide container, add enough slices to cover the brownies completely. Seal the container shut.
Leave the brownies at room temperature overnight, or up to a full day. The brownies will extract and soak up the moisture from the sliced bread, making them soft again. If the brownies are not sufficiently soft when you check them, replace the old bread slices with fresh slices and wait another several hours before checking again.