Take a slice of an apple or an apple core and place it in an airtight container or a gallon-sized zipper bag with hardened brown sugar. Making sure the apple is in contact with the sugar.
Close the container or bag and let it sit for at least 3 hours.
After 3 hours, check the sugar. If it is still hard, leave the apple and sugar in the container or bag for longer.
Once the sugar is soft and pliable, it is ready to use for baking.